On Super-Earths, Sub-Neptunes and Some Lessons They Teach

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Part 2 of 2

The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)
The Kepler-452 system compared alongside the Kepler-186 system and our solar system. Kepler-186 is a miniature solar system that would fit entirely inside the orbit of Mercury. The size of the habitable zone of star Kepler-452, considered one of the most “Earth-like” exoplanets found so far, is nearly the same as that of our sun. “Super-Earth” Kepler-452b orbits its star once every 385 days. (NASA Ames/JPL-CalTech/R. Hurt)

 

With such a large proportion of identified exoplanets in the super-Earth to sub-Neptune class, an inescapable question arises: how conducive might they be to the origin and maintenance of life?

So little is actually know about the characteristics of these planets that are larger than Earth but smaller than Neptune (which has a radius four times greater than our planet) that few are willing to offer a strong opinion.

Nonetheless, there are some seemingly good reasons to be optimistic, about the smaller super-Earths in particular. And there are some seemingly good reasons to be pessimistic –many appear to be covered in a thick layer of hydrogen and helium gas, with a layer of sooty smog on top, and that does not sound like an hospitable environment at all.

But if twenty years of exoplanet hunting has produced any undeniable truth, it is that surprising discoveries are a constant and overturned theories the norm. As described in Tuesday’s post, it was only several years ago that results from the Kepler Space Telescope alerted scientists to the widespread presence of these super-Earths and sub-Neptunes, so the fluidity of the field is hardly surprising.

One well-respected researcher who is bullish on super-Earth biology is Harvard University astronomy professor Dimitar Sasselov. He argues that the logic of physics tells us that the “sweet spot” for planetary habitability is planets from the size of Earth to those perhaps as large as 1.4 Earth radii. Earth, he says, is actually small for a planet with life, and planets with a 1.2 Earth radii would probably be ideal.

I will return to his intriguing analysis, but first will catalog a bit of what scientists have detected or observed so far about super-Earths and sub-Neptunes. As a reminder, here’s the chart of Kepler exoplanet candidate and confirmed planets that orbit G, K and M main sequence stars put together by Mission Scientist for the Kepler Space Telescope Kepler Natalie Batalha.

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Kepler exoplanets candidates, both confirmed and unconfirmed, orbiting G, K, and M type main sequence stars, by radii and fraction of the total. (Natalie Batalha and Wendy Stenzel, NASA Ames)

Expanding a bit:

  • While very large exoplanets were the first to be found because of the kind of instruments and techniques used in the search, the consensus view now is that small planets are much more plentiful. Future instruments will doubtless reveal a vast collection of exoplanets in the Earth-radius ballpark and smaller, but super-Earths and sub-Neptunes may well still dominate.
  • There’s a growing body of evidence that when planets are larger than roughly 1.5 Earth radii, they will likely be surrounded by a hyrogen and helium envelope dating back to the formation of the planet. The pull of the larger planets keeps the gases intact and often will render the planet essentially inert.
  • Roughly 70 percent of the main sequence stars in the galaxy are in the M dwarf category, smaller and less powerful stars that are known to have many exoplanets. Often they orbit close in to their sun and are packed together in a tight habitable zone. But many M dwarf planets in their habitable zones are like our moon – tidally locked so one side always faces the sun. Whether or not that rules them out in terms of habitability is now a hotly debated topic.

It’s quite amazing what has been learned about super-Earths, but compared to our knowledge of our solar system planets, we know very, very little. And what, after enormous effort, imagination and cost we do know? Generally speaking, gross measurements of mass, size, orbit period and density. They can tell scientists a lot about super-Earths and larger exoplanets, such as whether they are rocky, gaseous, and the mixtures in between. But characterizing them – and ultimately determining if they are at all capable of supporting life — really requires at a minimum that ability to measure what elements and compounds are in its atmosphere.

There are techniques for doing this: When an exoplanet passes in front of its star, the chemical make-up of the planet’s atmosphere can be analyzed by looking at how light either passes through or is absorbed by molecules, providing a telltale spectral reading of its contents.

trans_spec
Probing potassium in the atmosphere of HD 80606b with tunable filter transit spectrophotometry from the Gran Telescopio Canarias (European Space Observatory.)

Caroline Morley, of Jonathan Fortney’s group at the University of California at Santa Cruz, is one of those working to understand those measurements. But so far, the larger super-Earth and sub-Neptune planets are not cooperating.

“No spectral features are coming through, and so we’re limited in our characterizing,” Morley said. “This is a fundamental problem.” The spectral blanks, she and others are convinced, are the result of either thick clouds (probably made up of salts like zinc sulfide and potassium chloride) or a sooty hydrocarbon smog surrounding the planets. They keep the necessary stellar light from passing through in a way that would allow the presence of an enriched atmosphere, if present, to be identified and analyzed.

Like Morley, Robert Charnay and Victoria Meadows of the Virtual Planetary Laboratory at the University of Washington have been working to understand the opaque nature of the sub-Neptune planet Gliese 1214b in particular. They used a 3D circulation model to determine that salt clouds, which would form at lower altitudes, could nonetheless rise to the upper atmosphere and block any spectral readings.

These obstacles to analyzing the atmospheres of super-Earths were an initial surprise and have been a major frustration in the field. The James Webb Space Telescope may be able to peer through the clouds via detection of thermal emissions, so the 2018 arrival of the successor to the Hubble is eagerly awaited.

Caroline Morey of the University of California, Santa Cruz
Caroline Morey of the University of California, Santa Cruz (Jennifer Burt)

Based on what astronomers, planetary scientists, astrophysicists and others have been able to learn so far about the super-Earths to sub-Neptunes, the picture for potential habitability does not appear particularly bright. But there are other ways to assess that informal class of planets and come up with very different conclusions.

Dimitar Sasselov looks at astronomical problems from a more theoretical perspective, though he was a co-investigator for the Kepler telescope too. As Sasselov sees it, the basic physics of super-Earths, especially the smaller ones, actually favor life. The reason why is that it favors stability.

“There is no particular reason why a bigger planet might not be habitable,” he said. “When planets go bigger they get more stable, though certainly other problems will can and will arise. But when looking at some of the super-Earths, I contend they are as good for life as Earth, if not better.”

The additional stability comes in various forms. First is a less variable slant to the spin axis – its obliquity. Planets can get into trouble when that slant is highly changeable, as Sasselov says, pointing to Mars as an example. The planet’s steep and sometimes chaotic changes in the angle of its spin are believed to have caused dramatic climate changes.

Then there is the greater gravity that comes with a more massive planet. That increased gravity can have the effect of keeping an atmosphere from evaporating, a process that, among other things, exposes the planet to the charged particles coming from a parent star.

Dimitar Sallelov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative
Dimitar Sasselov, professor of Astronomy at Harvard University and director of the Harvard Origins of Life Initiative (NASA)

And there is the increased likelihood of some kind of recycling of material from the planet surface back into the mantle, where it gets chemically enriched. Plate tectonics allows for that kind of chemical redistribution on Earth, but smaller planets are not known to have parallel processes.

With a more stable atmosphere and planetary slant, “the geochemistry of a planet has plenty of time to leap to biochemistry, and then to adapt,” he said. “Life is rooted in geochemistry and has to play by the rules of chemistry and physics.   There are many aspects to stability, and all benefit from a planet being Earth-sized or larger.”

As for Earth itself, he sees our planet as being uncomfortably close to that low edge of what makes a planet not very stable – not especially habitable.

There are, of course, limits to finding habitable conditions on most large planets. Sasselov agrees that planets bigger than 1.5 earth radii or so will tend to keep their primordial envelope of hydrogen and helium, which can have the effect of freezing everything on the planet in place and making life impossible. They will also tend to be fully gaseous rather than rocky.

But there will always be outliers, he said, and he has spent a lot of time working on one of the – the “Mega-Earth” Kepler 10c. It is a huge super-Earth with a surprisingly high ratio of rock, orbiting its sun in 45 days and breaking all the rules – such as they are now – about planet formation.

An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)
An artist concept shows the Kepler-10 system, home to two rocky planets. In the foreground is Kepler-10c, a planet that weighs 17 times as much as Earth and is more than twice as large in size. This discovery has planet formation theorists challenged to explain how such a world could have formed. (Harvard-Smithsonian Center for Astrophysics/David Aguilar)

“We were very surprised to discover it,” said Sasselov, who was part of the Harvard-Smithsonian Center for Astrophysics team that first detected and analyzed it. “It was so large, so close to its sun, and much more massive than we would have predicted.”

It is a very hot planet and so not at all habitable, but it is a clear reminder that nature has ways of producing results that don’t appear at all plausible.

Sasselov said those anomalies may have to do with planet migration – that a large and gaseous Kepler 10c moved from the outer solar system into the close environs of its sun, and then lost some or all of its gas envelope from the heat and other solar activity. But the however it evolved – and since the system is more than 10 billion years old, it had a lot of time to evolve – it ended up a planet with 2.2 Earth radii with no apparent gas barriers around it and close to its sun. That, says Sasselov, makes it perfect to study with spectroscopy.

When NASA announced the huge mass of the planet in 2014, Kepler mission Batalha said: “Just when you think you’ve got it all figured out, nature gives you a huge surprise—in this case, literally.”

 

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Marc Kaufman

Marc Kaufman is the author of two books about space: “Mars Up Close: Inside the Curiosity Mission” and “First Contact: Scientific Breakthroughs in the Search for Life Beyond Earth.” He is also an experienced journalist, having spent three decades at The Washington Post and The Philadelphia Inquirer. While the “Many Worlds” column is supported and informed by NASA’s Astrobiology Program, any opinions expressed are the author’s alone.


To contact Marc, send an email to marc.kaufman@manyworlds.space.


2 thoughts on “On Super-Earths, Sub-Neptunes and Some Lessons They Teach

  1. Congratulations to the new blog! It has chosen an exciting topic.

    “But M dwarf planets in tat [sic] habitable zones are like our moon – tidally locked so one side always faces the sun. Whether or not that rules them out in terms of habitability is now a hotly debated topic.”

    This often repeated blanket claim makes me uncomfortable, because we know from our own system that it is blatantly untrue. The first image shows why, Kepler-186f orbits an M dwarf at teh corresponding orbit that Mercury does around a much more massive star, yet Mercury is not 1:1 locked but in a 3:2 spin-orbit resonance. [ https://en.wikipedia.org/wiki/Mercury_(planet)#Orbit.2C_rotation.2C_and_longitude ]

    I’m an interested layman, not an orbital mechanic, Jim. But that is obviously not only a failed test of the claim, but implies there must be a fair likelihood of at least the 3:2 resonance occurring. (Larger than 5 % one would hope, or it would conflict with our usual definition of requirement for likely enough observations.)

    It seems the mechanisms are numerous and arguable. Mercury is predicted to have had a 50 % likelihood of 3:2 capture due to chaotic evolution of eccentricity under the influence of outer planets. [ https://www.ncbi.nlm.nih.gov/pubmed/15215857 ] Kepler-186f on the other hand is claimed to have the same likelihood, but from another mechanism, perhaps the one that the above reference does away with because “such a process requires very specific values of the core viscosity”. [ http://www.seti.org/hangout/planet-in-habitable-zone ]

    If the mechanism is the more likely eccentric forcing, that may be frequent if single planet systems are those that have ejected other planets leaving their own path open to a 50 % likelihood of capture in a resonant orbit. Then up to 50 % of habitable planets around M stars could be not 1:1 tidally locked. And given that those stars are 80 % of stars, that makes a whole lot of non-tidal locked planets in the radiative HZ.

    The above SETI Institute video makes an interesting observation. Jill Tarter says roughly transcribed that the SETI community uncritically claimed for years that habitable planets around M stars would be tidally locked “like the Moon”. They only revisited the topic when such planets were found to be numerous. Maybe that could be generalized to cover the early years of the whole astrobiological community?

  2. Further theoretical work by Valencia and others suggests that super-Earths would be more geologically active than Earth, with more vigorous plate tectonics due to thinner plates under more stress. In fact, their models suggested that Earth was itself a “borderline” case, just barely large enough to sustain plate tectonics.

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