Weird Planets

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Artist rendering of an “eyeball world,” where one side of a tidally locked planet is always hot on the sun-facing side and the back side is frozen cold.  Definitely a tough environment, but  might some of the the planets be habitable at the edges?  Or might winds carry sufficient heat from the front to the back?  (NASA/JPL-Caltech)

The very first planet detected outside our solar system powerfully made clear that our prior understanding of what planets and solar systems could be like was sorely mistaken.

51 Pegasi was a Jupiter-like massive gas planet, but it was burning hot rather than freezing cold because it orbited close to its host star — circling in 4.23 days.  Given the understandings of the time, its existence was essentially impossible. 

Yet there it was, introducing us to what would become a large and growing menagerie of weird planets.

Hot Jupiters, water worlds, Tatooine planets orbiting binary stars, diamond worlds (later downgraded to carbon worlds), seven-planet solar systems with planets that all orbit closer than Mercury orbits our sun.  And this is really only a brief peak at what’s out there — almost 4,000 exoplanets confirmed but billions upon billions more to find and hopefully characterize.

I thought it might be useful — and fun — to take a look at some of the unusual planets found to learn what they tell us about planet formation, solar systems and the cosmos.

 


Artist’s conception of a hot Jupiter, CoRoT-2a. The first planet discovered beyond our solar system was a hot Jupiter similar to this, and this surprised astronomers and led to the view that many hot Jupiters may exist. That hypothesis has been revised as the Kepler Space Telescope found very few distant hot Jupiters and now astronomers estimate that only about 1 percent of planets are hot Jupiters. (NASA/Ames/JPL-Caltech)

 

Let’s start with the seven Trappist-1 planets.  The first three were detected two decades ago, circling a”ultra-cool” red dwarf star a close-by 40 light years away.  Observations via the Hubble Space Telescope led astronomers conclude that two of the planets did not have hydrogen-helium envelopes around them, which means the probability increased that the planets are rocky (rather than gaseous) and could potentially hold water on their surfaces.

Then in 2016 a Belgian team, using  the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile, found three more planets, and the solar system got named Trappist-1.  The detection of an additional outer planet was announced the next year, and in total three of the seven planets were deemed to be within the host star’s habitable zone — where liquid water could conceivably be present.

So, we have a most interesting 7-planet solar system quite close to us, and not surprisingly it has become the focus of much observation and analysis.

But consider this:  all seven of those planets orbits Trappist-1 at a distance much smaller than from our sun to the first planet, Mercury. The furthest out planets orbits the star in 19 days, while Mercury orbits in 88 days.

 

 

The Trappist-1 solar system, with the transit data used to detect the presence of seven planets, each one blocking the light curve at different locations. (NASA/JPL-Caltech)

 

Given this proximity, then, why are the Trappist-1 planets so interesting, especially in terms of habitability?  Because Trappist-1 puts out but .05 percent as much energy as our sun, and the furthest out planet (though very close to the star by the standards of our solar system) is nonetheless likely to be frozen.

So Trappist-1 a mini-system, with seven tidally-locked (never-rotating) planets that happen to orbit in resonance to each other.  Just because it is so different from our system doesn’t mean it isn’t fascinating, instructive, and even possibly the home of planets that could potentially support life.

And since red dwarf stars are the most common type of star in the Milky way (by lot), red dwarf solar system research is an especially hot field.

So there are mini planets and systems and massive planets in what used to be considered the impossibly wrong place.  And then there are planets with highly eccentric orbits — very different from the largely circular orbits of planets in our system.

The eccentricity of HD20782b superimposed onto our circular-orbiting inner solar system planets. (Stephen Kane)

The most extreme eccentric orbit found so far is HD 20782, measured at an eccentricity of .96. This means that the planet moves in a nearly flattened ellipse, traveling a long path far from its star and then making a fast and furious slingshot around the star at its closest approach. 

Many exoplanets have eccentricities far greater than what’s found in our solar system planets but nothing like this most unusual traveler, which has a path seemingly more like a comet than a planet.

Researchers have concluded that the eccentricity of a planet tends to relate to the number of planets in the system, with many-planeted systems having far more regularly orbiting planets.  (Ours and the Trappist-1 system are examples.)

Unusual planets come in many other categories, such as the chemical makeup of their atmospheres, surfaces and cores.  Most of the mass of stars, planets and living things consists of hydrogen and helium, with oxygen, carbon, iron and nitrogen trailing far behind.

Solid elements are exceptionally rare in the overall scheme of the solar system. Despite being predominant on Earth, they constitute less than 1 percent of the total elements in the solar system, primarily because the amount of gas in the sun and gas giants is so great.  What is generally considered the most important of these precious solid elements is iron, which is inferred to be in the core of almost all terrestrial planet.

The amount of iron or carbon or sulfur or magnesium on or around a planet generally depends on the amount of these “metals” present in the host star, and then in molecular protoplanetary disc remains of the star’s formation.  And this is where some of the outliers, the apparent oddities, come in.

A super-Earth, planet 55 Cancri e, was reported to be the first known planet to have huge layers of diamond, due in part to the high carbon-to-oxygen ratio of its host star. That conclusion has been disputed,  but the planet is nonetheless unusual.  Above is an artist’s concept of the diamond hypothesis. (Haven Giguere/Yale University)

The planet 55 Cancri e, for instance, was dubbed a “diamond planet” in 2012 because the amount of carbon relative to oxygen in the star appeared to be quite high.  Based on this measurement, a team hypothesized that the surface presence of abundant carbon likely created a graphite surface on the scalding super-Earth, with a layer of diamond beneath it created by the great pressures.

“This is our first glimpse of a rocky world with a fundamentally different chemistry from Earth,” lead researcher Nikku Madhusudhan of Yale University said in a statement at the time. “The surface of this planet is likely covered in graphite and diamond rather than water and granite.”

As tends to happen in this early phase of exoplanet characterization, subsequent measurements cast some doubt on the diamond hypothesis.  And in 2016, researchers came up with a different scenario — 55 Cancri e was likely covered in lava.  But because of heavy cloud and dust cover over the planet, a subsequent group raised doubts about the lava explanation. 

But despite all this back and forth, there is a growing consensus that 55 Cancri e has an atmosphere, which is pretty remarkable given its that its “cold” side has temperatures that average of 2,400 to 2,600 degrees Fahrenheit (1,300 to 1,400 Celsius), and the hot side averages 4,200 degrees Fahrenheit (2,300 Celsius). The difference between the hot and cold sides would need to be more extreme if there were no atmosphere.

 

Could super-Earth HD 219134 b be a sapphire planet? (Thibaut Roger/University of Zurich)

And then there’s another super-earth, HD 219134, that late last year was described as a planet potentially featuring vast collections of different precious stones.

To back up for a second, researchers study the formation of planets using theoretical models and compare their results with data from observations. It is known that during their formation, stars such as the sun were surrounded by a disc of gas and dust in which planets were born. Rocky planets like the Earth were formed out of the solid bodies left over when the protoplanetary gas disc cooled and dispersed.

Unlike the Earth however, HD 219134 most likely does not have a massive core of iron — a conclusion flowing from measurements of its density.  Instead, through modeling of formation scenarios for a scalding super-Earth close to its host star, the researchers conclude the planet is likely to be rich in calcium and aluminum, along with magnesium and silicon.

This chemical composition would allow the existence of large quantities of aluminum oxides. On Earth, crystalline aluminum oxide forms the mineral corundum. If the aluminum oxide contains traces of iron, titanium, cobalt or chromium, it will form the noble varieties of corundum, gemstones like the blue sapphire and the red ruby.

“Perhaps it shimmers red to blue like rubies and sapphires, because these gemstones are aluminum oxides which are common on the exoplanet,” said Caroline Dorn, astrophysicist at the Institute for Computational Science of the University of Zurich.

 

 

A variation on the “eyeball planet” is a water world where the star-facing side is able to maintain a liquid-water ocean, while the rest of the surface is ice. (eburacum45/DeviantArt)

 

Super-Earths, which are defined as having a size between that of Earth and Neptune, are also inferred to be the most likely to be water worlds.

At a Goldschmidt Conference in Boston last year, a study was presented that suggests that some super-Earth exoplanets are likely extremely wet with water – much more so than Earth. Astronomers found more specifically that exoplanets which are between two and four times the size of Earth are likely to have water as a dominant component.  Most are thought to be rocky and to have atmospheres, and now it seems that many have ocean, as well.

The new findings are based on data from the Kepler Space Telescope and the Gaia mission, which show that many of the already known planets of this type (out of more than 4,000 exoplanets confirmed so far) could contain as much as 50 percent water. That upper limit is an enormous amount, compared to 0.02 percent of the water content of Earth.

This potentially wide distribution of water worlds is perhaps not so surprising given conditions in our solar system, where Earth is wet, Venus and Mars were once wet, Neptune and Uranus are ice giants and moons such as Europa and Enceladus as global oceans beneath their crusts of ice.

 

Might this be the strangest planet of all? (NASA)

 

As is apparent with the planetary types described so far, whether a planet is typical or atypical is very much up in the air.  What is atypical this year may be found to be common in the days ahead.

The Kepler mission concluded that small, terrestrial planets are likely more common than gas giants, but our technology doesn’t let us identify and characterize many of those smaller, Earth-sized planets.

Many of the planets discovered so far are quite close to their host stars and thus are scalding hot. Planets orbiting red dwarf stars are an exception, but if you’re looking for habitable planets — and many astronomers are — then red dwarf planets come with other problems in terms of habitability.  They are usually tidally locked and they start their days bathed in very high-energy radiation that could stertilize the surface for all time.

A prime goal of the Kepler mission had been to find a planet close enough in character to Earth to be considered a twin.  While they have some terrestrial candidates that could be habitable, no twin was found.  This may be a function of lacking the necessary technology, or it’s certainly possible (if unlikely) that no Earth twins are out there.  Or at least none with quite our collection of conditions favorable to habitability and life. 

With this in mind, my own current candidate for an especially unusual planet is, well, our own.   Planet-hunting over the past almost quarter-century leads to that conclusion — for now, at least.

And it may be that solar systems like ours are highly unusual, too.  Pretty surprising, given that not long ago it was considered the norm.

 

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Does Proxima Centauri Create an Environment Too Horrifying for Life?

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Artist’s impression of the exoplanet Proxima Centauri b. (ESO/M. Kornmesser)

 

In 2016, the La Silla Observatory in Chile spotted evidence of possibly the most eagerly anticipated exoplanet in the Galaxy. It was a world orbiting the nearest star to the sun, Proxima Centauri, making this our closest possible exoplanet neighbour. Moreover, the planet might even be rocky and temperate.

Proxima Centauri b had been discovered by discerning a periodic wobble in the motion of the star. This revealed a planet with a minimum mass 30% larger than the Earth and an orbital period of 11.2 days. Around our sun, this would be a baking hot world.

But Proxima Centauri is a dim red dwarf star and bathes its closely orbiting planet in a level of radiation similar to that received by the Earth. If the true mass of the planet was close to the measured minimum mass, this meant Proxima Centauri b would likely be a rocky world orbiting within the habitable zone.

 

Comparison of the orbit of Proxima Centauri  b with the same region of the solar system. Proxima Centauri is smaller and cooler than the sun and the planet orbits much closer to its star than Mercury. As a result it lies well within the habitable zone. (ESO/M. Kornmesser/G. Coleman.)

Sitting 4.2 light years from our sun, a journey to Proxima Centauri b is still prohibitively long.

But as our nearest neighbor, the exoplanet is a prime target for the upcoming generation of telescopes that will attempt to directly image small worlds. Its existence was also inspiration for privately funded projects to develop faster space travel for interstellar distances.

Yet observations taken around the same time as the La Silla Observatory discovery were painting a very different picture of Proxima Centauri. It was a star with issues.

This set of observations were taken with Evryscope; an array of small telescopes that was watching stars in the southern hemisphere. What Evryscope spotted was a flare from Proxima Centauri that was so bright that the dim red dwarf star became briefly visible to the naked eye.

Flares are the sudden brightening in the atmosphere of a star that release a strong burst of energy. They are often accompanied by a large expulsion of plasma from the star known as a “coronal mass ejection”. Flares from the sun are typically between 1027 – 1032 erg of energy, released in a few tens of minutes.

For comparison, a hydrogen bomb releases the equivalent of about 10 megatons of TNT or a mere 4 x 1023 erg. Hitting the Earth, energy from solar flares and coronal mass ejections can disrupt communication equipment and create a spectacular aurora.

A solar flare erupting from the right side of the sun. (NASA/SDO)

But the Proxima super-flare spotted by Evryscope was well beyond a regular stellar flare.

On March 18 in 2016, this tiny red dwarf emitted an energy belch of 1033.5 erg. The flare consisted of one major event and three weaker ones and lasted approximately one hour, during which time Proxima Centauri became 68 times brighter.

A sudden, colossal increase in the brightness of a star does not bode well for any closely orbiting planets.

However, such a major flare might well be rare. If the star was normally fairly quiet, perhaps a planet could recover from a single very disruptive flare in the same way the Earth has survived mass extinction events.

Led by graduate student Ward Howard at the University of North Carolina, Chapel Hill, the discovering team used Evryscope to monitor Proxima Centauri for flares for a total of 1344 hours between January 2016 and March 2018. What they found was a horrifying environment, as reported in The Astrophysical Journal Letters.

While an event on the scale of the Proxima super-flare was only seen once, 24 large eruptions were spotted from the red dwarf, with energies from 1030.5 to 1032.4 erg. Allowing for the fact the star had only been observed for a small part of the year, this pattern of energy outbursts meant that a massive super-flare (1033 erg) was likely to occur at least five times annually.

 

Artist’s impression of the surface of the planet Proxima Centauri b. But what would conditions be like so close to a flaring star? (ESO/M. Kornmesser)

 

But how important is this for the planet?

The Earth is protected from flares from our sun by our atmosphere. The ozone layer absorbs harmful ultraviolet radiation with wavelengths between about 2400 – 2800 Angstroms (10-10 m), preventing it reaching the surface. So what if Proxima Centauri b had a similar protective layer of gases as the Earth?

To answer this question, Howard and his team ran simulations of an Earth-like atmosphere on Proxima Centauri b.

As is the case for the sun, the team assumed that large flares would be frequently accompanied by a coronal mass ejection. Radiation and stellar material then flooded over an Earth-like Proxima Centauri b at the observed rate. And the atmosphere crumbled.

 

Ward Howard, astrophysicist at the University of North Carolina.

High energy particles in the coronal mass ejections split the nitrogen molecules (N2) in the atmosphere, which reacted with the ozone (O3) to form nitrogen oxide (NO2). After just 5 years, 90% of the ozone in the atmosphere was lost and the amount was still decreasing.

Without ozone, the surface of Proxima Centauri b would be stripped of its protection from UV radiation. During the Proxima super-flare, the radiation dose without the protective ozone would be 65 times larger than that needed to kill 90% of one of the most UV-resilient organisms on Earth.

“Life would have to undergo extreme adaptation to UV or exist underground or underwater,” Howard notes. “Only the most resistant organisms could survive on the surface in this environment.”

The simulation does assume that Proxima Centauri b does not have a magnetic field. Such a shield could channel the particles from the coronal mass ejection to the poles, forming the aurora as on Earth and reducing the damage to the atmosphere.

However, orbiting so close to the star, Proxima Centauri b is likely to be in tidal lock as the moon is to the Earth. This is expected to weaken the magnetic field, as the slower rotation makes it harder to create a magnetic dynamo within the planet.

So if the protective shields are lowered on Proxima Centauri b, is our nearest planet a world populated by highly resistant UV organisms? Or have we seen evidence that rather than warming the planet to allow life to exist, this star has snuffed it out?

 

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Barnard’s Star, The “Great White Whale” of Planet Hunting, Has Surrendered Its Secret

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Barnard’s Star is the closest single star to our sun, and the most fast moving. It has long been attractive to planet hunters because it is so close and so bright, especially in the infared section of the spectrum. But until now, the exoplanets of this “great white whale” have avoided detection.

 

Astronomers have found that Barnard’s star — a very close, fast-moving, and long studied red dwarf — has a super-Earth sized planet orbiting just beyond its habitable zone.

The discovery relied on data collected over many years using the tried-and-true radial velocity method, which searches for wobbles in the movement of the host star.

But this detection was something big for radial velocity astronomers because Barnard-b was among the smallest planet ever found using the technique, and it was the furthest out from its host star as well — orbiting its star every 233 days.

For more than a century, astronomers have studied Barnard’s star as the most likely place to find an extrasolar planet.

Ultimately, said Ignasi Rablis of Spain’s Institute of Space Studies of Catalonia, lead author of the paper in journal Nature, the discovery was the result of 771 observations, an extremely high number.

And now, he said, “after a very careful analysis, we are over 99 percent confident the planet is there.”

The planet is at least 3.2 times the size of Earth and orbits near the snowline of the system, where water cannot be expected to ever be liquid.  That means is it a frozen world (an estimated -150 degrees Celsius) and highly unlikely to support life.

But Rablis and others on the large team say it also an extremely good candidate for future direct imaging and next-generation observing.

 

An artist’s rendering of the Barnard’s star planet at sunset. (Martin Kornmesser/ESO)

 

Thousands of exoplanets have been identified by now, and hundreds using the radial velocity method.  But this one is different.

“Barnard’s star is the ‘great white whale’ of planet hunting,” said Paul Butler, senior scientist at the Carnegie Institution, a radial velocity pioneer, and one of the numerous authors of the paper.

Because the star is so close (but 6 light-years away) and as a result so tempting, it has been the subject of exoplanet searches for 100 years, Butler said.  But until the radial velocity breakthroughs of the mid 1990s, the techniques used could not find a planet.

Nonetheless, an early exoplanet hunter, the Dutch-American astronomer Peter van de Kamp of Swarthmore College, thought that he had indeed found two gas giant planets around Barnard’s star in the 1960s.  He used a different technique based on the movement of the host star, and the findings even made it into some textbooks.  But later the detection was found to be incorrect.

Even after the modern exoplanet era began Barnard’s star kept its planetary secret close.

As Butler explained it, the combination of the planet’s size and distance from the star ultimately pushed the technology (and astronomers) to the very limit — requiring a measurement of  1.2 meters per second of “wobble.”

In contrast, the first planets were found by radial velocity that would detect 70 meter per second of wobble caused by the gravitational pull of a planet, and 30 years ago the best instruments could detect only 300 meters per second.

 

The radial velocity technique identifies planets via the shift in the wavelength of the light of a star as it wobbles due to the presence of a planet.  When a celestial object moves away from us, the light we observe becomes slightly less energetic and redder.  The opposite — light becomes slightly more energetic and bluer — happens when the star moves toward us.

 

The detected planet (which remains a “candidate” until further confirmed) was ultimately found following concerted effort by a large team of astronomers around the world.  It was co-led and organized by Guillem Anglada-Escudé of the Queen Mary University of London.  The young astronomer had made a major splash in 2016 with the detection of a planet orbiting Proxima Centauri, the closest star to our own.

That discovery was part of the “Pale Red Dot” campaign, which had the goal of detecting rocky planets around red dwarf stars.  After the Proxima discovery Barnard’s star went to the top of Anglada-Escudé list with the renamed “Red Dots” collaboration — which is supported by the European Southern Observatory and universities in Chile, the United Kingdom, Spain and Germany.

The Red Dots campaign is a collaboration including the European Southern Observatory, Queen Mary University of London, and several European and South American institutions.

By 2015, there was already almost 18 years of modern data collected regarding a possible planet orbiting the star, and a faint but clearly present signal had been detected.  But more was needed to confidently report a discovery, and the Red Dots effort took up the challenge.

To see if the result could be confirmed, astronomers regularly monitored Barnard’s star with high precision spectrometers such as the CARMENES (Calar Alto Observatory in Spain), and also the HARPS  (High Accuracy Radial velocity Planet Searcher.)

Ultimately, the team used observations from seven different instruments taken over 20 years, making this one of the largest and most extensive datasets ever used for precise radial velocity studies.

“We all have worked very hard on this result,” said Anglada-Escudé. “This is the result of a large collaboration organized in the context of the Red Dots project, which is why it has contributions from teams all over the world including semi-professional astronomers.”

Cristina Rodríguez-López, researcher at the Instituto de Astrofísica de Andalucía and co-author of the paper, said of the significance of the finding grow over decades.

Guillem Anglada-Escudé was a leader of the Barnard’s star collaboration, as he was in the successful campaign to detect a planet orbiting of Proxima Centauri.

“This discovery means a boost to continue on searching for exoplanets around our closest stellar neighbors, in the hope that eventually we will come upon one that has the right conditions to host life,” she said.

The next pr0ject for the Red Dots campaign is to study the star Ross 154, at 9.69 light-years away another of the closest stars to us.

The dramatically increased (and increasing) precision in radial velocity measurements is expected to continue with the next generation of ground-based telescopes and spectrometers.

Butler, for instance,  said that Carnegie is in the process of upgrading its Planet Finding Spectrograph at the Las Campanas Observatory in Chile to reach a 0.5-meters-per-second measurement. Other groups including the European Southern Observatory and American teams based at Pennsylvania State and Yale Universities have similar efforts under way.

If they succeed, Butler said, it may well be possible to find potentially habitable planets around sun-like and other categories of stars using the radial velocity method.

 

 

Barnard’s star is the fourth closest to our sun, and the closest single star. It lies 6 light-years from us, as opposed to a little more than 4 light-years for the Alpha Centauri/Proxima Centauri threesome. (NASA Photojournal)

 

Barnard’s a very-low-mass red dwarf star in the constellation of Ophiuchus. It is the fourth-nearest-known individual star to the sun (after the three components of the Alpha Centauri system) and the closest star in the Northern Celestial hemisphere.

Despite its proximity, the star is too faint to be seen with the unaided eye, though it is quite visible with an amateur 8-inch telescope.  It is much brighter in the infrared than in visible light.  Although Barnard’s Star is an ancient star, it still experiences star flare events, one being observed in 1998.

The star is named after the American astronomer E. E. Barnard.  He was not the first to observe the star (it appeared on Harvard University plates in 1888 and 1890), but in 1916 he measured its proper motion –the apparent angular motion of a star across the sky with respect to more distant stars — as 10.3 arcseconds per year relative to the sun.

This is likely to be the fastest star in terms of proper motion, as its proximity to the sun, as well as its high velocity, make it unlikely any faster object will be discovered.

Barnard’s Star is among the most studied red dwarfs because of its proximity and favorable location for observation near the celestial equator. Historically, research on Barnard’s Star has focused on measuring its stellar characteristics and its astrometry — which involves precise measurements of the positions and movements of stars and other celestial bodies on the plane of the sky.

When planet hunters use astrometry, they look for a minute but regular wobble in a star’s position as seen in images.  Van de Kamp, for instance, used astrometry to study Barnard’s star and (incorrectly) detected those two gas giants around it.

In contrast, radial (or Doppler) velocities look for the wobble of the star perpendicular to the plane sky, and astronomers have regularly, and now once again, made history with that method.

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What Would Happen If Mars And Venus Swapped Places?

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Venus, Earth and Mars (ESA).

 

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.

 

The classical habitable zone around our sun marks where an Earth-like planet could support liquid water on the surface (Cornell University).

 

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,” suggests Chris Colose, a climate scientist based at the NASA Goddard Institute for Space Studies and who proposed the topic for discussion.

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 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.

 

The surface of Mars captured by a selfie taken by the Curiosity rover at a site named Mojave. (NASA/JPL-Caltech/MSSS.)

 

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.

Artist’s rendering of a solar storm hitting Mars and stripping ions from the planet’s upper atmosphere (credit: NASA/GSFC).

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.”

 

The surface of Venus captured from the former Soviet Union’s Venera 13 spacecraft, which touched down in March 1982. (NASA)

 

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.

The thick atmosphere of Venus captured by the Akatsuki orbiter. (JAXA)

“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 vapour 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.

“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,” Colose said. “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.”

 

Artist impression of a terraformed Mars. (NASA GSFC)

 

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 (credit: NASA/Goddard Space Flight Center Scientific Visualization Studio).

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.

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!”

 

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Large Reservoir of Liquid Water Found Deep Below the Surface of Mars

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Artist impression of the Mars Express spacecraft probing the southern hemisphere of Mars, superimposed on a radar cross section of the southern polar layered deposits. The leftmost white line is the radar echo from the Martian surface, while the light blue spots are highlighted radar echoes along the bottom of the ice.  Those highlighted areas measure very high reflectivity, interpreted as being caused by the presence of water. (ESA, INAF. Graphic rendering by Davide Coero Borga )

Far beneath the frigid surface of the South Pole of Mars is probably the last place where you might expect the first large body of Martian liquid water would be found.  It’s -170 F on the surface, there are no known geothermal sources that could warm the subterranean ice to make a meltwater lake, and the liquid water is calculated to be more than a mile below the surface.

Yet signs of that liquid water are what a team of Italian scientists detected — a finding that they say strongly suggests that there are other underground lakes and streams below the surface of Mars.  In a Science journal article released today, the scientists described the subterranean lake they found as being about 20 kilometers in diameter.

The detection adds significantly to the long-studied and long-debated question of how much surface water was once on Mars, a subject that has major implications for the question of whether life ever existed on the planet.

Finding the subterranean lake points to not only a wetter early Mars, said co-author Enrico Flamini of the Italian space agency, but also to a Mars that had a water cycle that collected and delivered the liquid water.  That would mean the presence of clouds, rain, evaporation, rivers, lakes and water to seep through surface cracks and pool underground.

Scientists have found many fossil waterways on Mars, minerals that can only be formed in the presence of water, and what might be the site of an ancient ocean.

But in terms of liquid water now on the planet, the record is thin.  Drops of water collected on the leg of NASA’s Phoenix Lander after it touched down in 2008, and what some have described as briny water appears to be flowing down some steep slopes in summertime.  Called recurrent slope lineae or RSLs, they appear at numerous locations when the temperatures rise and disappear when they drop.

This lake is different, however, and its detection is a major step forward in understanding the history of Mars.

Color photo mosaic of a portion of Planum Australe on Mars.  The subsurface reflective echo power is color coded and deep blue corresponds to the strongest reflections, which are interpreted as being caused by the presence of water. (USGS Astrogeology Science Center, Arizona State University, INAF)

The discovery was made analyzing echoes captured by the the radar instruments on the European Space Agency’s Mars Express, a satellite orbiting the planet since 2002.  The data for this discovery was collected from observation made between 2012 and 2015.

 

A schematic of how scientists used radar to find what they interpret to be liquid water beneath the surface of Mars. (ESA)

Antarctic researchers have long used radar on aircraft to search for lakes beneath the thick glaciers and ice layers,  and have found several hundred.  The largest is Lake Vostok, which is the sixth largest lake on Earth in terms of volume of water.  And it is two miles below the coldest spot on Earth.

So looking for a liquid lake below the southern pole of Mars wasn’t so peculiar after all.  In fact, lead author Roberto Orosei of the Institute of Radioastronomy of Bologna, Italy said that it was the ability to detect subsurface water beneath the ice of Antarctica and Greenland that helped inspire the team to look at Mars.

There are a number of ways to keep water liquid in the deep subsurface even when it is surrounded by ice.  As described by the Italian team and an accompanying Science Perspective article by Anja Diez of the Norwegian Polar Institute, the enormous pressure of the ice lowers the freezing point of water substantially.

Added to that pressure on Mars is the known presence of many salts, that the authors propose mix with the water to form a brine that lowers the freezing point further.

So the conditions are present for additional lakes and streams on Mars.  And according to Flamini, solar system exploration manager for the Italian space agency, the team is confident there are more and some of them larger than the one detected.  Finding them, however, is a difficult process and may be beyond the capabilities of the radar equipment now orbiting Mars.

 

Subsurface lakes and rivers in Antarctica. Now at least one similar lake has been found under the southern polar region of Mars. (NASA/JPL)

The view that subsurface water is present on Mars is hardly new.  Stephen Clifford, for many years a staff scientist at the Lunar and Planetary Institute, even wrote in 1987 that there could be liquid water at the base of the Martian poles due to the kind of high pressure environments he had studied in Greenland and Antarctica.

So you can imagine how gratifying it might be to learn, as he put it “of some evidence that shows that early theoretical work has some actual connection to reality.”

He considers the new findings to be “persuasive, but not definitive” — needing confirmation with other instruments.

Clifford’s wait has been long, indeed.  Many observations by teams using myriad instruments over the years did not produce the results of the Italian team.

Their discovery of liquid water is based on receiving particularly strong radar echoes from the base of the southern polar ice — echoes consistent with the higher radar reflectivity of water (as opposed to ice or rock.)

After analyzing the data in some novels ways and going through the many possible explanations other than the presence of a lake, Orosei said that none fit the results they had.  The explanation, then, was clear:  “We have to conclude there is liquid water on Mars.”

The depth of the lake — the distance from top to bottom — was impossible to measure, though the team concluded it was at least one meter and perhaps in the tens of meters.

Might the lake be a habitable?  Orosei said that because of the high salt levels “this is not a very pleasant environment for life.”

But who knows?  As he pointed out, Lake Vostok and other subglacial Antarctic lake, are known to be home to single-cell organisms that not only survive in their very salty world, but use the salt as part of their essential metabolism.

 

 

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