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The question How are rogue planets discovered? describes the difficulties in finding planets (or planet-sized objects) that are floating through space without being under the influence of any star or Galaxy system.
What would the possible surface conditions of these planets be? Could chemotrophic life arise on such planets?
Rogue planets have two formation mechanisms: Independent formation and ejection.
An independently formed rogue planet would have condensed out of the nebular material by itself and not formed from a young star's protoplanetary disk. We understand how individual stellar-mass objects condense and which have been tested by observation, but I'm not aware of any tested studies which predict rate at which Earth-massed objects independently form. (We certainly have not observed a large population of them, and the gravitational micro-lensing studies which have been done would have detected them if they existed.) But since Brown Dwarfs are fairly common, it seems safe to assume that there is a population of super-Jupiters that formed by direct condensation.
The planets formed by ejection should range in size from meteors right up to super-Jupiters and should be broadly similar to the size distribution of bound planets. (If small planets tend to be closer to their star, there may be some bias against them being ejected.)
Planets are mostly ejected early in a system's life, but it can happen at any point in a star's lifetime -- planetary system dynamics never becomes perfectly stable. And if there's a near passage of another star or a large rogue planet, ejections can occur even after billions of years of stability. (See Fritz Leiber's "A Pail of Air"!) But the population of ejected planets is probably very heavily weighted towards planets ejected soon after formation.
The distinction between independently formed and ejected planets is important because when a planet forms it is very hot from the gravitational energy released in formation, and most likely initially inimical to life forming. But it cools (the surface much faster than the interior) and eventually life can form.
If the planet remains in orbit around a star, the surface temperature drops asymptotically towards an equilibrium temperature where the sum of the star's radiational input and the heat leakage from the still-hot interior balances the IR radiation leaked by the planet into space. Frequently, that will be in a temperature range where life can form on the surface.
Once a planet is ejected, the equilibrium temperature will be much lower. For example, for Earth at 4.5 GY old, the Sun's radiation dumps 3000 times as much energy onto the Earth's surface than does leakage from the interior, (see https://en.wikipedia.org/wiki/Earth%27s_internal_heat_budget for details.) The time it takes for an Earth-sized rogue planet to cool to Earth's current temperature is on the order of 10 MY and after that it would just keep on cooling down to a surface temperature on the order of 30K.
So the question of life forming on an Earth-like planet is one of whether or not there is time before the surface layers freeze solid, since that would appear to halt the formation of life.
All bets are off for larger planets which (a) cool more slowly and (b) probably have enough more water to have subsurface liquid oceans even after the surface freezes solid.
For planetary bodies where life does have time to form, the only source of energy would be the heat leaking from the interior either directly due to the heat gradient (a very diffuse source), or indirectly from the equivalent of Earth's deep sea vents. The latter seems more likely since the steeper heat and chemical potential gradients are a lot easier to exploit.
Hal Clement wrote two scientifically excellent stories set on such planets. One, Star Light, has high-gravity aliens working with humans to explore the surface of Dhrawn, a Brown Dwarf. The other was short fiction and had humans meeting intelligent life living on an closer to Earth-sized rogue planet. (I think it was "Sortie" and sequels, but I'm not sure.)
In any event, it seems likely that there would be a great variety of different kinds of rogue planet that might potentially support life.
David Stevenson has theorized that a rouge planet could be ejected with considerable hydrogen in the atmosphere that would lead to a high-pressure hydrogen atmosphere. This is highly opaque to infrared and could conceivably hold the little bit of internally-generated heat well enough that water could exist on the surface. Here's an abstract of that paper: Life-sustaining Planets in Interstellar Space.
During planet formation, rock and ice embryos of the order of Earth's mass may be formed, some of which may be ejected from the Solar System as they scatter gravitationally from proto-giant planets. These bodies can retain atmospheres rich in molecular hydrogen which, upon cooling, can have basal pressures of 102 to 104 bars. Pressure-induced far-infrared opacity of H2 may prevent these bodies from eliminating internal radioactive heat except by developing an extensive adiabatic (with no loss or gain of heat) convective atmosphere. This means that, although the effective temperature of the body is around 30 K, its surface temperature can exceed the melting point of water. Such bodies may therefore have water oceans whose surface pressure and temperature are like those found at the base of Earth's oceans. Such potential homes for life will be difficult to detect.
Another such paper is: Constraints on the free-floating planets supporting aqueous life, by Viorel Badescu.
Billions of Undetected 'Rogue Planets' Could Be Tearing Wildly Through Our Galaxy
We tend to think of planets neatly ordered in systems, like our own Solar System. But every now and then, astronomers get hints of something else - rogue planets, not attached to any star or system, drifting lonely through the galaxy.
Now it turns out there could be many more of them than anyone ever suspected. A new simulation has revealed that there could be billions of rogue planets in the Milky Way.
Taking a census of exoplanets is an extraordinarily tricky business at the best of times, when they're in a planetary system. Planets don't give off any light of their own (unless we're faced with a brown dwarf, but their planet status is debatable), so we have to use other means of detecting them.
When a planet is orbiting a star, there are two main detection methods. These are the radial velocity method, whereby the planet's gravitational effect on the star can be detected, and the transit method, when the planet orbits in front of the star and slightly dims its light.
As you can see, both of these methods rely on the presence of a star - so they're pretty useless when it comes to rogue planets. Nevertheless, there are still some ways we can detect these wanderers. Two rogue planets were spotted last year because of the way their gravity bends light coming from behind them infrared imaging has revealed others.
In all, around 20 rogue planets or candidate rogue planets have been identified, compared to 3,917 exoplanets in planetary systems. That's a pretty huge gap.
So, to find out how many rogue planets might be out there, drifting darkly through the galaxy, astronomers at the University of Leiden in The Netherlands ran sophisticated mathematical simulations of the Orion Trapezium, a cluster of young stars in the heart of the Orion Nebula.
Five hundred Sun-like stars were modelled with four, five, or six planets each, for a total of 2,522 planets in all, with masses ranging from around three times that of Earth to around 130 times the mass of Jupiter (that's brown dwarf-sized).
Of these planets, 357 (16.5 percent) became unbound from their systems within 11 million years of forming, and drifted free. A few stayed within the cluster, and five were captured by other planetary systems, but 282 - the majority of the planets - escaped the cluster completely.
Interestingly, 75 of the 2,522 collided with their host star, and 34 planets collided with another planet.
Most of the rogue planets we've actually observed (which is a very small number) have been on the larger side. But the researchers found that a planet's mass was unlikely to have an effect on the likelihood it would be ejected from a system.
So rogue planets probably run the same gamut of sizes as planets bound by a system. There are probably smaller rogue planets out there - they're just that much harder to detect, among an already hard-to-detect class of objects.
If the 16.5 percent figure can be broadly extrapolated across the Milky Way (which is possible, given the Trapezium Cluster is pretty typical of a stellar nursery), then there are at least 16.5 billion rogue planets wandering around, of the estimated total of at least 100 billion planets.
It's even possible that our own system had an extra planet once upon a time that got booted out. It's thought that Uranus' weird axial tilt could have been caused by a collision with a rogue planet, and it's even possible that the mysterious and hypothetical Planet Nine was a rogue that got captured as it passed by.
As the researchers note, the study is based on only two simulations, so it's certainly not definitive - but the simulation evidence does indicate rogue planets are much more common than what our observational evidence suggests.
And it certainly sparks the imagination - these cold, lonely planets, traversing space far from the warmth of a star. Someone should definitely write a science fiction movie about that.
The research has been accepted into Astronomy & Astrophysics, and can be read on arXiv.
Are there planets orbiting stars in between galaxies?
I heard that they are lone stars that are in between galaxies and I was wondering if it was possible for these stars to be solar systems. If yes, is it also possible for these planets to be habitable?
Yes, some of those rogue stars will have planets, and there's no reason why those planets couldn't be habitable.
Some rogue stars are ejected by three-body interactions with supermassive black holes which give them an enormous velocity and send them flying into intergalactic space. These systems might have been gravitationally disrupted during the interaction, which could disrupt planetary orbits and strip them from the system entirely.
Other rogue stars are ejected through tidal interactions between galaxies. This is a much gentler process as far as the stellar system is concerned, and probably will not put the planetary system in any real danger. These planets could certainly be habitable. In fact, being away from the galaxy lowers your chances of getting hit by a nearby supernova.
Massive ‘rogue’ planet with mysterious ‘glow’ discovered outside solar system – and it’s 12 times larger than Jupiter
A HUGE "rogue" planet with an unexplained "glow" lurks beyond our solar system, claim scientists.
The monstrously large world is 12 times bigger than gas giant Jupiter and the first object of its kind to be spotted using a radio telescope, according to the National Radio Astronomy Observatory.
They're dubbing it a "rogue" because it's mysteriously "drifting" through space without any kind of orbit around a parent star.
More baffling still is its mass and powerful magnetic field that's over 200 times stronger than Jupiter's.
Picking apart its secrets could lead to the discovery of more alien worlds, claim the boffs.
“This object is right at the boundary between a planet and a brown dwarf, or ‘failed star,’ and is giving us some surprises that can potentially help us understand magnetic processes on both stars and planets,” said Dr Melodie Kao, an astronomer at Arizona State University.
Brown dwarves have long stumped scientists: they're too huge to be considered planets and not big enough to be considered stars.
They also have strong auroras – similar to the stunning "Northern Lights" on Earth – like those seen in our own solar system's giant planets Jupiter and Saturn.
The auroras on our planet are caused by its magnetic field interacting with the solar wind (the continuous flow of charged particles from the sun's upper atmosphere, known as the corona, that permeates the solar system).
Kao's team used an advanced radio telescope located in New Mexico to make the discovery. They say the new world is 200 million years old and 20 light-years from Earth.
It also boasts scorching surface temperatures of around 825 degrees Celsius, or more than 1,500 degrees Farenheit.
By comparison, the Sun’s surface temperature is about 5,500 degrees Celsius.
Though it was first detected in 2016, scientists initially identified it as one of five recently discovered brown dwarfs.
That theory was scrapped, however, after they dug through more data to better pinpoint its age.
They now believe it's a much younger object and its mass is therefore smaller than originally thought – meaning it could theoretically be classified as a planet in its own right.
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GONE WITHOUT I-TRACE
Such a strong magnetic field “presents huge challenges to our understanding of the dynamo mechanism that produces the magnetic fields in brown dwarfs and exoplanets and helps drive the auroras we see,” said Gregg Hallinan, of Caltech.
He added: “Detecting SIMP J01365663+0933473 with the VLA through its auroral radio emission also means that we may have a new way of detecting exoplanets, including the elusive rogue ones not orbiting a parent star.”
“This particular object is exciting because studying its magnetic dynamo mechanisms can give us new insights on how the same type of mechanisms can operate in extrasolar planets — planets beyond our Solar System," added Kao.
"We think these mechanisms can work not only in brown dwarfs, but also in both gas giant and terrestrial planets."
‘Lonely’ Rogue Planet Discovered Wandering the Milky Way
Researchers believe that our galaxy is teeming with cosmic orphans, planets wandering free of a parent star. Though common, these rogue planets are difficult to spot, especially when they are in the size range of the earth.
Despite this difficulty an international team of astronomers including Przemek Mróz, a postdoctoral scholar at the California Institute of Technology (Caltech) and Radosław Poleski from the Astronomical Observatory of the University of Warsaw, have spotted what they believe to be a free-floating planet with a size and mass somewhere in the range of Mars and Earth, wandering the Milky Way.
The discovery represents a major step forward in the field of exoplanet investigation as it is the first earth-sized ‘rogue planet’ ever observed.
An artist’s impression of a gravitational microlensing event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)
“We found a planet that seems extremely lonely and small, far away in the Universe,” Poleski tells ZME Science. “If you can imagine, Earth is in a sandbox surrounded by lots of other planets, and light from the Sun. This planet isn’t. It’s truly alone.”
The rogue planet the team found — OGLE-2016-BLG-1928 — is believed to be the smallest free-floating planet ever discovered. It was found in data collected by Optical Gravitational Lensing Experiment (OGLE), a Polish astronomical project based at the University of Warsaw. Previously discovered rogues — such as the first-ever recorded free-floating planet also found by OGLE in 2016 — are closer in size to Jupiter.
The gravity of a free-floating planet may deflect and focus light from a distant star when passing close in front of it. Due to the distorted image, the star temporarily seems much brighter. (v)
“We discovered the smallest free-floating planet candidate to date. The planet is likely smaller than Earth, which is consistent with the predictions of planet-formation theories,” Mróz — lead author of the team’s study published in Astrophysical Journal Letters — explains to ZME Science. “Free-floating planets are too faint to be observed directly — we can detect them using gravitational microlensing via their light-bending gravity.”
The Gravity of the Situation
The team spotted this wandering planet using the technique of gravitational microlensing, often utilised to spot exoplanets — planets outside our solar system. Exoplanets can’t often be observed directly, and when they can it’s a result of interaction with radiation from their parent star — for example, the dimming effect exoplanets have when they cross in front of their star and block some of the light it emits. Clearly, as rogue planets don’t have a parent star, they don’t have these interactions, making micro-lensing events the only way of spotting them.
“Microlensing occurs when a lensing object — a free-floating planet or star — passes between an Earth-based observer and a distant source star, its gravity may deflect and focus light from the source,” Mróz explains to ZME Science. “The observer will measure a short brightening of the source star, which we call a gravitational microlensing event.”
When the gravity of a free-floating planet deflects and focuses light from a distant star, we can observe temporary changes in star brightness. (temporary changes in star brightness.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw.)
Mróz continues by explaining that the duration of microlensing events depends on the mass of the object acting as a gravitational lens. “The less massive the lens, the shorter the microlensing event. Most of the observed events, which typically last several days, are caused by stars,” Mróz says. “Microlensing events attributed to free-floating planets usually last barely a few hours which makes them difficult to spot. We need to very frequently observe the same part of the sky to spot brief brightenings caused by free-floating planets.”Changes of brightness of the observed star during the gravitational microlensing event by a free-floating planet. (Credit: Jan Skowron / Astronomical Observatory, University of Warsaw/ Robert Lea)
By measuring the duration of a microlensing event and shape of its light curve astronomers can estimate the mass of the lensing object. That is how the team were able to ascertain this free-floating planet is approximately Earth-sized. “Hence, we can discover very dim objects, like black holes, or free-floating planets,” says Poleski. “We found it an event, which has a timescale of 41 minutes. And it’s the shortest event ever discovered.”
Poleski explains that the lack of any other lensing body in the system told the team that it is a very strong candidate for a free-floating planet. He adds: “We know it’s a planet because of the very short timescale and we think it’s free-floating because we don’t see any star next to it.”
Going Rogue. How Free-Floating Planets Come to Wander the Universe Alone
Astronomers believe that free-floating planets actually formed in protoplanetary disks around stars in the same way that ‘ordinary’ planets are. At some point, they are ejected from their parent planetary systems, probably after gravitational interactions with other bodies, for example, with other planets in the system.
“Some low-mass planets are expected to be ejected from their parent planetary systems during the early stages of planetary system formation,” says Mróz. “According to planet formation theories, most of the ejected planets should be smaller than Earth. Theories of planet formation predict that typical masses of ejected planets should be between 0.3 and 1.0 Earth masses. Thus, the properties of this event fit the theoretical expectations.”
These free-floating rogue planets are believed to be fairly common, but researchers can’t be certain because they are so difficult to spot. “Our current studies indicate that the frequency of low-mass–in the Earth to super-Earth-mass range–free-floating or wide-orbit planets is similar to that of stars — there are about two-five such objects per each star in the Milky Way,” says Mróz. “These numbers are very uncertain because they are based on a few sightings of short-timescale microlensing events. However, if free-floating/wide-orbit planets were less frequent than stars, we would have observed much fewer short-timescale events than we do.”
The researcher adds that though these objects are relatively common, the chances of observing microlensing events caused by them are still extremely small. “Three objects — source, lens, and observer — must be nearly perfectly aligned,” Mróz says. “If we observed only one source star, we would have to wait almost a million year to see the source being microlensed.”
In fact, one of the extraordinary elements of the team’s study is that such a short duration lensing event wasn’t believed to be observable given the sensitivity of the current generation of telescopes.
“The surprise, in general, was that with current technology we could define such a short time event,” Poleski says. “It’s especially surprising if you beat the previous record by a factor of few.”
The Nancy Grace Roman Telescope and Future Rogue Reconnaissance
For Mróz, there are still questions that he would like to see answered about OGLE-2016-BLG-1928. Primarily, confirming that it definitely is a free-floating planet.
“We aren’t fully sure whether our planet is free-floating or not. Our observations rule out the presence of stellar companions within 10 astronomical units million miles–of the planet, but the planet may have a more distant companion,” Mróz says. “Let’s imagine that we’re observing microlensing events by a doppelganger of the Solar System. If Jupiter or Saturn caused a microlensing event, we would see a signature of the Sun in the microlensing event light curve. However, microlensing events by Uranus or Neptune would likely look like those of free-floating planets, because they are very far from the Sun.”
Fortunately, Mróz says that should be possible to distinguish between free-floating and wide-orbit planets. “The lens is moving relative to the source star in the sky and — a few years after the microlensing event — the lens and source should separate in the sky,” the researcher elaborates. “If the lens has a stellar companion, we will see some excess of light at its position. If it is a free-floating planet, we will not.”
Whilst this method may seem simple, Mróz says we cannot apply it now, because the existing telescopes are not powerful enough. This includes the instrument that conducted the long-term observations that gave rise to the OGLE sky survey–the data from which the team found the micro-lensing event OGLE-2016-BLG-1928.
“[The discovery of OGLE-2016-BLG-1928] was part of the larger search for microlensing events in general, which we perform in a number of steps,” Poleski tells ZME. “In one step, I started looking at the wide orbit planets — planets similar to Uranus, or Neptune and on similar orbits. And while looking for those, I screened a list of candidate microlensing events in general and I found this one.”
Soon NASA’s Nancy Grace Roman Telescope will take over the search for microlensing events, but in the meantime, there is still data from OGLE and other projects to be examined. “We now have more data and other surveys are also collecting data. So we hope to analyze those,” Poleski says. “The longer-term future is the launch of the Nancy Grace Roman Space Telescope. It will be a telescope similar to the Hubble telescope, only with new infrared and infrared cameras and that camera field of view larger than the Hubble Space Telescope.
“One of the main projects for the Raman telescope will be to observe galactic bulge in search for microlensing planets, including free-floating planets.”
Mroz, P., Poleski, R., Gould, A. et al., ‘A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event,’ Astrophysical Journal Letters,  DOI: 10.3847/2041–8213/abbfad
A strange, lonely planet found without a star
An international discovered a young, exotic, rogue planet – PSO J318.5-22, is just 80 light-years away from Earth and has a mass six times that of Jupiter it was formed apporximately 12 million years ago – which makes it a newborn in terms of planets (the Earth was formed approximately 4.5 billion years ago.
Multicolor image from the Pan-STARRS1 telescope of the free-floating planet PSO J318.5-22, in the constellation of Capricornus. Most of its energy is emitted in infrared.
“We have never before seen an object free-floating in space that that looks like this. It has all the characteristics of young planets found around other stars, but it is drifting out there all alone,” explained team leader Dr. Michael Liu of the Institute for Astronomy at the University of Hawaii at Manoa. “I had often wondered if such solitary objects exist, and now we know they do.”
During the past decade, the discovery of new exoplanet has developed at an exponential pace, without about 1.000 (!) new planet discovered through indirect methods. However, even with the astonishing development of technique, only a handful were observed through direct imaging.
“Planets found by direct imaging are incredibly hard to study, since they are right next to their much brighter host stars. PSO J318.5-22 is not orbiting a star so it will be much easier for us to study. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter shortly after their birth,” said Dr. Niall Deacon of the Max Planck Institute for Astronomy in Germany and a co-author of the study.
Astronomers have confirmed the existence of rogue planets only a few year ago, so this is also an exciting, new field of study. There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.
An Interstellar Visitor Had a Sad Story to Tell
A hint of warmth from our sun helped reveal a mysterious comet’s secrets.
In 2019, Gennady Borisov, an amateur astronomer in Crimea, discovered his seventh comet. This icy object wasn’t like the others Borisov had found, or like any of the other comets in the solar system. This one wasn’t orbiting the sun.
Instead, it had been drifting alone in interstellar space, following its own path, until one day, it entered our solar system and grazed past the sun. Warmed by the heat of a star, for the first time in who knows how long, the icy comet thawed just a little bit.
Some of Earth’s most powerful telescopes captured the cosmic interloper as it went by. Astronomers could see the comet enveloped in a fuzzy glow of once-frozen dust particles loosed by the sun.
By analyzing these particles from afar, researchers have managed to learn about the comet’s composition, its origins, and its long journey here. One recent finding demonstrates something rather melancholy.
Of the comets astronomers have observed, this one—named Borisov, after its discoverer—is one of the most pristine. “Think of the wind erosion of the mountains, or even the suntan on our skin when we go to the beach,” Stefano Bagnulo, an astronomer at Armagh Observatory, in the United Kingdom, who had studied Borisov, told me. Borisov shows very few signs of another sunny encounter in its journey through space. For a comet to be as unblemished as this one means it has been extremely alone.
Astronomers had already assumed that, given the vast distances between stars, objects like Borisov can travel for eons without running into another one. The last time Borisov felt the warmth of a star was probably in the bounds of its own system. Astronomers can retrace Borisov’s journey only so far, and we’ll likely never know where the comet came from. But the shimmery cloud of particles surrounding Borisov, known as a coma, can tell us something. “Dust carries rich information about the planetary system,” Bin Yang, an astronomer at the European Southern Observatory, in Chile, told me.
In another recent analysis of Borisov’s dust particles, Yang and her team found evidence that suggests the comet formed close to its parent star before spinning into the outer parts of its system, gathering up different kinds of cosmic material as it went along. Yang says Borisov might owe its composition to the presence of giant planets, which are known for stirring things up with their gravity. Perhaps Borisov once shared a home with its own versions of Jupiter and Saturn.
Though Borisov’s arrival was a surprise, the comet is less mysterious than ‘Oumuamua, the first interstellar visitor ever detected, seen in 2017. At the time, astronomers had been expecting something that resembled Borisov current theories suggest that icy comets near the edges of a planetary system can be jostled by big planets and flung out into an untethered existence in the space between stars. Our solar system, in its early, more chaotic days, probably kicked out a few comets of its own. But ‘Oumuamua looked more like an asteroid, and astronomers are still debating its exact nature, including its shape. Given two very different interstellar visitors, the astronomy community is eager to see what’s next, and it won’t have to wait long. A new observatory in Chile expected to excel at spotting interstellar objects will begin operations in 2022, and the European Space Agency plans to launch a set of spacecraft in 2029 that will idle in space until they’re commanded to chase after a newly found interstellar object.
Borisov has now left us behind, traveling beyond the view of any telescopes. It will not leave our solar system in the condition it arrived. Last spring, as Borisov neared Jupiter, the Hubble space telescope captured imagery that showed a piece of the comet breaking off. “It is not pristine anymore,” Ludmilla Kolokolova, an astronomer at the University of Maryland who worked with Bagnulo, told me. Borisov now bears a mark of its visit through the solar system. Which presents an interesting question: Hypothetically, if Borisov were to pass by another star, where a set of alien astronomers could observe it, would they notice any evidence of its encounter with our sun?
Bagnulo said it’s difficult to say whether Borisov’s trip through the solar system noticeably altered the cometary properties he studied, which, presumably, his hypothetical alien counterparts might also investigate. But when a comet moves away from the sun and cools off, Kolokolova said, some of the particles in its coma can return to the surface and harden into a crust. “If this comet goes to another system where astronomers look at it, they will see it was heated,” Kolokolova said. “They wouldn’t know if it was the sun or any other star, but they could see the comet was heated.”
But Borisov is unlikely to skim by another star. More than one astronomer told me that the chances are nearly zero. The distances between stars are simply too big. “If you had a collision between the Milky Way and another Milky Way, you could collide the galaxies and no two stars would ever hit,” David Jewitt, an astronomer at UCLA who studies comets, told me. Astronomers believe Borisov coasted alone for hundreds of millions of years, even billions, through space before reaching us. “In that amount of time, you might pass by one star,” Jewitt said. “So for Borisov, maybe this is it.”
For us, the fleeting experience was illuminating. Our interstellar guest gave us evidence of its own home. There, too, the cosmos had struck a match, igniting a star into existence and leaving just enough kindling behind to make planets and moons. The process has unfolded countless times across the universe, creating islands of clustered matter, isolated far from one another. Through a chance encounter with a comet like Borisov, we can glimpse some of the ways these alien places might resemble our own.
Surface of the Planets
People have been intrigued for centuries by whether life could exist on other planets. While we now know that it is very unlikely that life as we know it could exist on other planets in our Solar System, many people do not know the surface conditions of these various planets.
Mercury resembles nothing so much as a larger version of the Moon. This planet is so close to the Sun that it is actually difficult to observe. The Hubble Space Telescope cannot look at it because it would permanently damage the lens.
Venus’ atmosphere of thick, toxic clouds hides the planet’s surface from view. Scientists and amateurs alike used to think that the planet was covered with thick forests and flora like tropical rainforests on Earth. When they were finally able to send probes to the planet, they discovered that Venus’ surface was actually more like a vision of hell with a burning landscape that is dotted with volcanoes.
Mars has very diverse terrain. One of the planet’s most famous features is its canals, which early astronomers believed were “man”-made and contained water. These huge canyons were most likely formed by the planet’s crust splitting. Mars is also famous for its red color, which is iron oxide (rust) dust that covers the surface of the entire planet. The surface of Mars is covered with craters, volcanoes, and plains. The largest volcanoes of any planet are on Mars.
Jupiter is a gas giant, so it has no solid surface just a core of liquid metals. Astronomers have created a definition for the surface – the point at which the atmosphere’s pressure is one bar. This region is the lower part of the atmosphere where there are clouds of ammonia ice.
Saturn is also a gas giant so it has no solid surface only varying densities of gas. Like Jupiter, almost all of Saturn is composed of hydrogen with some helium and other elements in trace amounts.
Uranus and Neptune are also gas giants, but they belong to the subcategory of ice giants because of the “ices” in their atmospheres. Uranus’ surface gets its blue color from the methane in the atmosphere. Methane absorbs light that is red or similar to red on the color spectrum leaving only the light near the blue end of the spectrum visible.
Neptune is also blue due to the methane in its atmosphere. Its “surface” has the fastest winds of any planet in the Solar System at up to 2,100 kilometers per hour.
Universe Today has a number of articles including surface of Mars and surface of Mercury.
NASA’s Timely Question –“If Venus Switched Places With Mars, Would It Be Habitable?”
“When I suggested this topic, I wondered whether two inhabited planets would exist (the Earth and Venus) if Mars and Venus formed in opposite locations,” said Chris Colose, a climate scientist based at the NASA Goddard Institute for Space Studies. “Being at Mars’s orbit would avoid the runaway greenhouse and a Venus-sized planet wouldn’t have its atmosphere stripped as easily as Mars.”
What would happen if you switched the orbits of Mars and Venus? Would our solar system have more habitable worlds?
It was a question raised at the “Comparative Climatology of Terrestrial Planets III” a meeting held in Houston at the end of August. It brought together scientists from disciplines that included astronomers, climate science, geophysics and biology to build a picture of what affects the environment on rocky worlds in our solar system and far beyond.
The question regarding Venus and Mars was proposed as a gedankenexperiment or “thought experiment” a favorite of Albert Einstein to conceptually understand a topic. Dropping such a problem before the interdisciplinary group in Houston was meat before lions: the elements of this question were about to be ripped apart.
The Earth’s orbit is sandwiched between that of Venus and Mars, with Venus orbiting closer to the sun and Mars orbiting further out. While both our neighbors are rocky worlds, neither are top picks for holiday destinations.
Mars has a mass of just one-tenth that of Earth, with a thin atmosphere that is being stripped by the solar wind a stream of high energy particles that flows from the sun. Without a significant blanket of gases to trap heat, temperatures on the Martian surface average at -80°F (-60°C). Notably, Mars orbits within the boundaries of the classical habitable zone (where an Earth-like planet could maintain surface water) but the tiny planet is not able to regulate its temperature as well as the Earth might in the same location.
Unlike Mars, Venus has nearly the same mass as the Earth. However, the planet is suffocated by a thick atmosphere consisting principally of carbon dioxide. The heat-trapping abilities of these gases soar surface temperatures to above a lead-melting 860°F (460°C).
But what if we could switch the orbits of these planets to put Mars on a warmer path and Venus on a cooler one? Would we find that we were no longer the only habitable world in the solar system?
“Modern Mars at Venus’s orbit would be fairly toasty by Earth standards,” Colose. Dragging the current Mars into Venus’s orbit would increase the amount of sunlight hitting the red planet. As the thin atmosphere does little to affect the surface temperature, average conditions should rise to about 90°F (32°C), similar to the Earth’s tropics. However, Mars’s thin atmosphere continues to present a problem.
Colose noted that without a thicker atmosphere or ocean, heat would not be transported efficiently around Mars. This would lead to extreme seasons and temperature gradients between the day and night. Mars’s thin atmosphere produces a surface pressure of just 6 millibars, compared to 1 bar on Earth. At such low pressures, the boiling point of water plummets to leave all pure surface water frozen or vaporized.
Mars does have ice caps consisting of frozen carbon dioxide, with more of the greenhouse gas sunk into the soils. A brief glimmer of hope for the small world arose in the discussion with the suggestion these would be released at the higher temperatures in Venus’s orbit, providing Mars with a thicker atmosphere.
However, recent research suggests there is not enough trapped carbon dioxide to provide a substantial atmosphere on Mars. In an article published in Nature Astronomy, Bruce Jakosky from the University of Colorado and Christopher Edwards at Northern Arizona University estimate that melting the ice caps would offer a maximum of a 15 millibars atmosphere.
The carbon dioxide trapped in the Martian rocks would require temperatures exceeding 300°C to be liberated, a value too high for Mars even at Venus’s orbit. 15 millibars doubles the pressure of the current atmosphere on Mars and surpasses the so-called “triple point” of water that should permit liquid water to exist. However, Jakosky and Edwards note that evaporation would be rapid in the dry martian air. Then we hit another problem: Mars is not good at holding onto atmosphere.
Orbiting Mars is NASA’s Mars Atmosphere and Volatile Evolution Mission (MAVEN). Data from MAVEN has revealed that Mars’s atmosphere has been stripped away by the solar wind. It is a problem that would be exacerbated at Venus’s orbit.
“Atmospheric loss would be faster at Venus’s current position as the solar wind dynamic pressure would increase,” said Chuanfei Dong from Princeton University, who had modeled atmospheric loss on Mars and extrasolar planets.
This “dynamic pressure” is the combination of the density of particles from the solar wind and their velocity. The velocity does not change greatly between Mars and Venus —explained Dong— but Venus’s closer proximity to the sun boosts the density by almost a factor of 4.5. This would mean that atmosphere on Mars would be lost even more rapidly than at its current position.
“I suspect it would just be a warmer rock,” Colose concluded.
While Mars seems to fare no better at Venus’s location, what if Venus were to be towed outwards to Mars’s current orbit? Situated in the habitable zone, would this Earth-sized planet cool-off to become a second habitable world?
Surprisingly, cooling Venus might not be as simple as reducing the sunlight. Venus has a very high albedo, meaning that the planet reflects roughly 75% of the radiation it receives. The stifling temperatures at the planet surface are due not to a high level of sunlight but to the thickness of the atmosphere. Conditions on the planet may therefore not be immediately affected if Venus orbited in Mars’s cooler location.
“Venus’s atmosphere is in equilibrium,” pointed out Kevin McGouldrick from the University of Colorado and contributing scientist to Japan’s Akatsuki mission to explore Venus’s atmosphere. “Meaning that its current structure does depend on the radiation from the sun. If you change that radiation then the atmosphere will eventually adjust but it’s not likely to be quick.”
Exactly what would happen to Venus’s 90 bar atmosphere in the long term is not obvious. It may be that the planet would slowly cool to more temperate conditions. Alternatively, the planet’s shiny albedo may decrease as the upper atmosphere cools. This would allow Venus to absorb a larger fraction of the radiation that reached its new orbit and help maintain the stifling surface conditions. To really cool the planet down, Venus may have to be dragged out beyond the habitable zone.
“Past about 1.3 au, carbon dioxide will begin to condense into clouds and also onto the surface as ice,” said Ramses Ramirez from the Earth-Life Sciences Institute (ELSI) in Tokyo, who specializes in modelling the edges of the habitable zone. (An “au” is an astronomical unit, which is the distance from our sun to Earth.)
Once carbon dioxide condenses, it can no longer act as a greenhouse gas and trap heat. Instead, the ice and clouds typically reflect heat away from the surface. This defines the outer edge of the classical habitable zone when the carbon dioxide should have mainly condensed out of the atmosphere at about 1.7 au. The result should be a rapid cooling for Venus. However, this outer limit for the habitable zone was calculated for an Earth-like atmosphere.
“Venus has other things going on in its atmosphere compared to Earth, such as sulphuric acid clouds,” noted Ramirez. “and it is much drier, so this point (where carbon dioxide condenses) may be different for Venus.”
If Venus was continually dragged outwards, even the planet’s considerable heat supply would become exhausted.
“If you flung Venus out of the solar system as a rogue planet, it would eventually cool-off!” pointed out Max Parks, a research assistant at NASA Goddard.
It seems that simply switching the orbits of the current Venus and Mars would not produce a second habitable world. But what if the two planets formed in opposite locations? Mars is unlikely to have fared any better, but would Venus have avoided forming its lead-melting atmosphere and become a second Earth?
At first glance, this seems very probable. If the Earth was pushed inwards to Venus’s orbit, then water would start to rapidly evaporate. Like carbon dioxide, water vapor is a greenhouse gas and helps trap heat. The planet’s temperature would therefore keep increasing in a runaway cycle until all water had evaporated. This “runaway greenhouse effect” is a possible history for Venus, explaining its horrifying surface conditions. If the planet had instead formed within the habitable zone, this runaway process should be avoided as it had been for the Earth.
But discussion within the group revealed that it is very hard to offer any guarantees that a planet will end up habitable. One example of the resultant roulette game is the planet crust. The crust of Venus is a continuous lid and not series of fragmented plates as on Earth. Our plates allow a process known as plate tectonics, whereby nutrients are cycled through the Earth’s surface and mantle to help support life. Yet, it is not clear why the Earth formed this way but Venus did not.
One theory is that the warmer Venusian crust healed breaks rapidly, preventing the formation of separate plates. However, research done by Matt Weller at the University of Texas suggests that the formation of plate tectonics might be predominantly down to luck. Small, random fluctuations might send two otherwise identical planets down different evolutionary paths, with one developing plate tectonics and the other a stagnant lid. If true, even forming the Earth in exactly the same position could result in a tectonic-less planet.
A rotating globe with tectonic plate boundaries indicated as cyan lines. Venus’s warmer orbit may have shortened the time period in which plate tectonics could develop, but moving the planet to Mars’s orbit offers no guarantees of a nutrient-moving crust.
A rotating globe with tectonic plate boundaries indicated as cyan lines. (NASA/Goddard Space Flight Center)
Yet whether plate tectonics is definitely needed for habitability is also not known. It was pointed out during the discussion that both Mars and Venus show signs of past volcanic activity, which might be enough action to produce a habitable surface under the right conditions.
Of course, moving a planet’s orbit is beyond our technological abilities. There are other techniques that could be tried, such as an idea by Jim Green, the NASA chief scientist and Dong involving artificially shielding Mars’s atmosphere from the solar wind.
“We reached the opposite conclusion to Bruce’s paper,” Dong noted cheerfully. “That is might be possible to use technology to give Mars an atmosphere. But it is fun to hear different voices and this is the reason why science is so interesting!”
Outcast Planets Could Support Life
If aliens exist, where are they? Many astronomers look to the nearest stars, in the hope that they harbor a warm, wet planet like Earth. But now a pair of researchers believe extraterrestrial life could exist on a rogue planet that has been ejected from its birthplace.
Astronomers have never spotted a rogue planet with certainty, but computer simulations suggest that our galaxy could be teeming with them. Slingshotted out of their planetary system by the gravity of a bigger planet, these lone worlds zoom far from their parent suns, slowly freezing in the cold of outer space. Any water fit for life would freeze, too. Yet in a paper submitted to The Astrophysical Journal Letters, planetary scientists Dorian Abbot and Eric Switzer of the University of Chicago in Illinois suggest that a rogue planet could support a hidden ocean under its blanket of ice, kept warm by geothermal activity.
They call such a world a Steppenwolf planet after a novel by the German-Swiss author Hermann Hesse, because "any life . would exist like a lone wolf wandering the galactic steppe." If Steppenwolf planets do exist, there's a chance that some of them could be lurking in space between Earth and nearby stars. If so, they might be a more realistic human destination for the search of alien life than another planetary system, which would be at least several light-years away. There is even a chance—albeit very small—that a Steppenwolf planet crashing into our solar system billions of years ago was the origin of life on Earth.
Abbot and Switzer came to their conclusion by simulating an isolated planet between 1/10th and 10 times the size of Earth. By comparing the rate at which heat would be lost through an ice shell with the rate at which heat would be produced by geothermal activity, they calculated that a planet with Earth's composition of rock and water but three times as big would generate enough heat to maintain a hidden ocean. If the planet had much more water than Earth, say Abbot and Switzer, it would need to be only about a third as big as our planet. "Several kilometers of water ice make an excellent blanket that could be sufficient to support liquid water at its base," says Switzer.
The Chicago researchers are not the first to consider the possibility of liquid water on rogue planets. In 1999, planetary scientist David Stevenson of the California Institute of Technology in Pasadena, calculated that liquid water could exist if a planet had a dense atmosphere of hydrogen—so dense that a greenhouse effect would trap warmth on the surface without the need for ice. But Abbot thinks the new result is more surprising because they are considering a more generic planet, without an extraordinary atmosphere.
"This is certainly an interesting study regarding the extent of the possible locations where life could arise, or be sustained, in the universe," says David Ehrenreich, a planetary scientist at the Joseph Fourier University in Grenoble, France. "However, it will certainly be very difficult to actually detect life on such a world, since it would be buried under an ice shell."
Switzer admits detection would be difficult. An astronomer would need to spot a Steppenwolf planet by looking for its infrared emission to see if it is as warm as he and Abbot predict. But at present, even the best observatories could detect rogue planets only within about 100 billion miles of Earth—not a huge distance in astronomical terms—and Switzer says the probability of a Steppenwolf planet existing in this range is just one in a billion.
Still, as planetary scientist Gaetano Di Achille of the University of Colorado, Boulder, points out, that might mean that the first occupied planet humans set foot on is not in another planetary system, but in the lonely depths of outer space. "If the hypothesis of oceans on rogue planets is correct, we will certainly have to expand the inventory of places with a high potential for life," he says.