Super-earth or mini-Neptune? Telling habitable worlds apart from lifeless gas giants

Super-earth or mini-neptune? Telling habitable worlds apart from lifeless gas giants
A super-Earth basks in the glow of its star in this artist's impression. Is the world habitable and Earth-like with a watery atmosphere, or is it Neptune-esque, swathed in hydrogen and helium? Credit: ESO/M. Kornmesser

Perhaps the most intriguing exoplanets found so far are those bigger than our rocky, oceanic Earth but smaller than cold, gas-shrouded Uranus and Neptune. This mysterious class of in-between planets—alternatively dubbed super-Earths or mini-Neptunes—confounds scientists because nothing like them exists as a basis for comparison in our solar system.

"We don't really know what they are," said Björn Benneke, a graduate student in astronomy at the Massachusetts Institute of Technology. "They can be a scaled-down version of the giant planets in our , a scaled-up version of terrestrial planets like Earth, or something completely different."

Benneke is co-author of a paper accepted by the Astrophysical Journal that attempts to solve this vexing riddle. Based on numerical computer models, he developed an observational strategy that would let astronomers distinguish between two very different types of atmospheres associated with these planets. Learning about their atmospheres will speak to the overall nature of these heretofore unknowable worlds with masses ranging up to about 10 times that of Earth. (Uranus and Neptune have 14 and 17 Earth-masses, respectively.)

A tale of two atmospheres

The first scenario is an atmosphere dominated by hydrogen and helium, like that of Uranus and Neptune. The second is an atmosphere composed predominately of larger compounds, such as water vapor, carbon dioxide and nitrogen molecules, like Earth, or carbon monoxide and methane, among others.

The essential difference between these two scenarios goes beyond satisfying mere curiosities of planetary science. Figuring out if an exoplanet is more like Neptune or more like Earth drives at one of the core conceits of all astronomical work: the search for alien life.

Lacking a rocky surface and oceans, a gaseous mini-Neptune would not be hospitable to life as we know it. On the other hand, a super-Earth with water and other biologically enabling chemistries could serve as a thriving extraterrestrial abode.

Crucially, the discernment between mini-Neptunes and super-Earths as proposed by Benneke can be done with available telescopes. Astronomers and the public will therefore not have to wait until next-generation instruments such as the James Webb Space Telescope come online later this decade.

"What is nice about this is that the proposed observations are possible with current instruments," he said. "They would represent the first observational baby step to saying something definitive about what the conditions are on these planets."

Benneke and colleagues at the University of Chicago have in fact already been allotted observational time with the Hubble Space Telescope to stare long and hard at the exoplanet GJ 1214b. They hope to apply their mathematical approach and be able to state, even as soon as this year, what the basic character of this world is.

Super-earth or mini-neptune? Telling habitable worlds apart from lifeless gas giants
Earth on the left and an artist's impression of a super-Earth on the right. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

Scientists have debated over GJ 1214b since the planet was discovered back in 2009. Based on its size and mass—2.8 Earth radii and 6.6 Earth masses—GJ 1214b could be covered in oceans hundreds of miles deep, or a thick envelope of hydrogen and helium gas might instead surround its rocky core.

Sniffing out an atmosphere

Benneke's concept for characterizing atmospheres relies on planetary transits, which is when an exoplanet crosses in front of the star as seen from Earth and blocks some of the star light during this transit. The Kepler mission has looked at more than a hundred thousand stars for this slight dimming effect that reveals the presence of planets and some of their basic properties. Other space telescopes, such as Hubble and the Spitzer Space Telescope, also make use of this technique.

When a transit occurs, starlight shines through the exoplanet's atmosphere. Different wavelengths of light preferentially pass through this atmosphere based on its composition, thickness, cloud content and so on.

By simultaneously measuring how much the star becomes fainter during the planet's transit at different wavelengths, the general extent of the planet's atmosphere and by extension its chemical character can be inferred.

Benneke found that while it is possible to infer the presence of gases like water vapor and carbon dioxide from this spectrum, determining the relative amounts of the gases is trickier. Puffy, hydrogen-rich atmospheres with a minute amount of water vapor and high-altitude clouds can display water absorption features of the same strength as atmospheres made entirely of water vapor.

Fortunately, visualizing the data can help. When the transmission spectrum is plotted on a graph, the steepness of the absorption features can be a tell-tale sign of the amount of the atmosphere's constituents. "When graphed, the spectrum of water-rich atmosphere appears wavier, but hydrogen-rich atmosphere show more distinct and spiky signatures," said Benneke. "This difference is unambiguous."

Effectively, the technique can be used to measure the average mass of the molecules in the exoplanet's atmosphere. And conveniently for the purposes of distinguishing a hydrogen-dominated atmosphere from a watery one, say, the molecular masses are strikingly different.

Super-earth or mini-neptune? Telling habitable worlds apart from lifeless gas giants
Basic schematic of a planetary transit. Instruments can register starlight passing through the planet's atmosphere that reveals the presence of certain gases. Credit: NASA Ames

Benneke offered an example. The average molecular mass for one of these super-Earth or mini-Neptune candidates could turn out to be about 18, say. Right away, Benneke said, such a figure would rule out a hydrogen-dominated atmosphere, the value of which would come in closer to two based on the mass of a hydrogen molecule, H2. Hydrogen consists of a single proton, which has a representative mass of one on the periodic table. (The electron associated with hydrogen has negligible mass.) A water molecule, H2O, has an atomic mass of 18 per the addition of oxygen's eight protons and eight neutrons. (Protons and neutrons have nearly the same mass and so register as one apiece in this calculation.)

"If we know the molecular mass is 18, we can already say the atmosphere can't be hydrogen-dominated," said Benneke. "On the opposite side, if it's somewhere around two or three, then we would know the planet must be like Neptune."

Putting exoplanets to the test

GJ 1214b stands as the ideal test case for Benneke's concept. The world tightly orbits a very small, dim star. Accordingly, the planet frequently crosses the face of its star with respect to our vantage point, offering astronomers plenty of data points. Furthermore, the planet's transits block a relatively large amount of its host star's light, providing astronomers a stronger signal with which to parse its atmosphere. "GJ 1214b as the first test case is by far the easiest to do," said Benneke.

Super-earth or mini-neptune? Telling habitable worlds apart from lifeless gas giants
Artist impressions of a variety of super-Earths. Credit: NASA Ames/JPL-Caltech

His technique could work for other transiting super-Earth and mini-Neptune candidates, some of which will sound familiar to exoplanet fans, including HD 97658b, 55 Cancri e, and GJ 436b.

The density of these worlds, like GJ 1214b, requires them to have more rocky or icy content than Uranus and Neptune, but not to be completely made of rock like Earth. Answering the question about whether the remainder of their composition is hydrogen and helium or life-friendly gases such as , carbon dioxide and nitrogen, will require substantially more observing time than for GJ 1412b, Benneke said, but is not impossible.

"We will be able to learn something about the atmospheres of those planets, and from there if they have a surface or if they are just some kind of gas giant," said Benneke. "There have been a lot of theoretical ideas of what these could be that would explain their mass and radius, but there hasn't been really an explanation of how we can unambiguously distinguish between the two scenarios of super-Earth and mini-Neptune, until now."

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Journal information: Astrophysical Journal

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Sep 30, 2013
From the article "What is nice about this is that the proposed observations are possible with current instruments"

That is not just nice but extraordinary. Many times we find we need more powerful interments to obtain more data. Benneke and his co-authors seem to have found a viable way of getting more data out of what we already have. The more we can do this the more we can learn.

I look forward to the results of this method used on GJ 1214b. My guess is it will be more like a mini Neptune. It would seem logical. Every planet larger than Earth we know of is a gas planet.

Sep 30, 2013
My guess is it will be more like a mini Neptune. It would seem logical.

The size versus density is a good indication that it's more like a large Venus. It's almost sure to have a rocky core. What we don't know is the chemistry of the atmosphere.

If we are talking about planets in the habitable zone, then I predict that surface gravity will be an important deciding factor. The surface gravity profoundly changes the chemistry in the atmosphere, since higher gravity leads to higher gas pressure. Higher gas pressure raises the temperature of phase changes, and should promote chemical reactions that form more complex molecules.

I think super earth sized planets are very likely to have complex atmospheres, with abundant heavy molecules like methane.

I'm not sure what that would mean for microbes. I guess that might depend on the pH of the atmosphere? But that's a bit out of my league.

A thick, complex atmosphere should block UV well, so that's a plus at the very least.

Sep 30, 2013
The size versus density is a good indication that it's more like a large Venus. It's almost sure to have a rocky core. What we don't know is the chemistry of the atmosphere.

There are theories for a rocky core in gas giants like Jupiter. I don't know how accurate the theories are. However, it would stand to reason on that basis for mini Neptune. The current assumption seems to be GJ 1214 b is a water world. No matter what the outcome of these observations it will be very interesting.

Sep 30, 2013
Ingenious and promising!

@Mr_Science: Not exactly "is a gas planet".

This year several results points to a dichotomy between planets with a radius less than twice Earth's and a radius larger than twice Earth's. People haven't been able to pinpoint it to the difference between superEarths and miniNeptunes because of confounds. (Say, the distance to the star - is the remaining atmosphere a heated hydrogen shell or not.) So it is better to say "we don't know".

(But that too looked cautiously promising.)

Sep 30, 2013
@GSwift7: If, as the evidence suggest, we evolved out of alkaline hydrothermal vent chemistry, the atmosphere conditions would be less important, I think.

It would certainly help if the atmosphere is reducing (hydrogen and/or methane) or make the oceans acidic anyway (carbon dioxide).

But the main redox source would be the thermal water cycling through rocks around the vent. If it had only local effect I assume it would set up a cell like (alkaline to acidic) potential somewhere in the crust. That would make life a more protracted and/or unlikely occurrence, since in an acidic ocean the deposited vent mineral "sponge" would supply the primary cellular compartments.

Ocean worlds would be an interesting study. In principle vents would work there too, at least until pressure icing sets in. How thick would the ocean need to be to have its ices cut off any geothermal activity?

Sep 30, 2013
"Telling habitable worlds apart from lifeless gas giants" - humanity must stop with Earth-centric life bearing assumptions. There is nothing in Physics or Chemistry that limits life to an Earth-like conditions. If anything, gas giants offer a diverse set of environments in which life can arise easier than on a small rocky planet.

Oct 01, 2013
Horrible title for this article. Who says gas giants must be 'lifeless'?? The chemistry there must be certainly as rich and complex as ours, we just don't know anything about it. Life is everywhere.

Oct 01, 2013
From a scientific standpoint the only life we know of exist on a rocky planetary body covered mostly by liquid water. While it is fair to say this is only one data point and we don't really know where life may exist. It is also fair to say, we can only look for life as we know it.

Life on gas planets may be a possibility. The author and scientist Arthur C Clarke played with the idea in his famous book 2001 Space Odyssey. It is possible we would not recognize it as life when we see it.

"Life is everywhere " is an opinion due to the lack of scientific evidence. While many would agree with the statement, there is currently no evidence for life anywhere except on Earth.

Oct 01, 2013
How thick would the ocean need to be to have its ices cut off any geothermal activity?

There are a lot of variables; too many really, and we don't know what happens when you 'turn the dials'.

Depending on surface gravity, the depth for icing would vary. Also, the amount of geological activity would matter. A sufficiently large volcano could build up through the ice layers.

Then there are factors we don't even understand here on Earth, such as the mechanisms behind ice ages and snowball earth periods.

I don't think its useful to contemplate exogenesis at this point. We will do well just to figure out what these smaller planets are made of first. Only after we know what they might look like can we begin to think about whether life has a chance there or not.

Oct 01, 2013
We also know that some clays and inorganic minerals can be powerful catalysts so solid surface and in particular coastlines may play a crucial role. On the other hand, dust particles in nebula are also very active reaction sites so perhaps dust in a gas giant atmosphere could do the same. The bottom line is that we don't know enough to say either way. We just keep looking at all the candidates but when observing time is limited, we naturally go for the safest bet first.

Oct 02, 2013
We just keep looking at all the candidates but when observing time is limited, we naturally go for the safest bet first

Our biggest limitation is simply being able to see them well enough. All of the Kepler targets are relatively far away, so there's not much we can see about most of them.

One way around that is to look at as many near-by planets as we can, and take note of any patterns that appear, such as x sized planet tends to be either a or b, then use that to extrapolate what the population of more distant planets might look like. That's the current plan for our next specialized planet hunting instrument. That's a LOT more time-consuming than staring at the same little patch of sky continuously, since it will need to point in a bunch of different directions to see all the nearby planets.

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