Are super-Earths actually mini-Neptunes?

Feb 04, 2013
A diagram comparing the Earth, at left, to a cross-section of a super-Earth on the right. The super-Earth has a relatively small rocky core, an atmosphere of methane, water and hydrogen and an extended hydrogen envelope. Credit: H. Lammer

(Phys.org)—In the last two decades astronomers have found hundreds of planets in orbit around other stars. One type of these so-called 'exoplanets' is the super-Earths that are thought to have a high proportion of rock but at the same time are significantly bigger than our own world. Now a new study led by Helmut Lammer of the Space Research Institute (IWF) of the Austrian Academy of Sciences suggests that these planets are actually surrounded by extended hydrogen-rich envelopes and that they are unlikely to ever become Earth-like. Rather than being super-Earths, these worlds are more like mini-Neptunes. The scientists publish their work in the journal Monthly Notices of the Royal Astronomical Society.

'Super-Earths' follow a different evolutionary track to the found in our Solar system but an open question is whether they can evolve to become rocky bodies like the '' Mercury, Venus, Earth and Mars. To try to answer this, Dr Lammer and his team looked at the impact of radiation on the upper atmospheres of super-Earths orbiting the stars -11, Gliese 1214 and .

This group of planets are all a few times more massive and slightly larger than the Earth. They orbit very close to their respective stars. The way in which the mass of planets scales with their sizes suggests that they have solid cores surrounded by hydrogen or hydrogen-rich atmospheres, probably captured from the clouds of gas and dust (nebulae) from which the planets formed.

The new model suggests that the short wavelength extreme ultraviolet light (much 'bluer' than the blue light we see with our eyes) of the host stars heats up the gaseous envelopes of these worlds, so that they expand up to several times the radius of each planet and gas escapes from them fairly quickly. Nonetheless most of the atmosphere remains in place over the whole lifetime of the stars they orbit.

An artist's impression comparing the super-Earth 55 Cancri e to the Earth. Credit: NASA/JPL-Caltech/R. Hurt (SSC)

"Our results indicate that, although material in the atmosphere of these planets escapes at a high rate, unlike lower mass Earth-like planets many of these super-Earths may not get rid of their -captured hydrogen-rich atmospheres", says Dr Lammer.

Rather than becoming more like the Earth, the super-Earths may more closely resemble Neptune, which together with Uranus, is a smaller 'gas giant' in our Solar system. If the scientists' results are right, then super-Earths further out from their stars in the 'habitable zone', where the temperature would allow liquid water to exist, would hold on to their atmospheres even more effectively. If that happens, they would be much less likely to be habitable.

The team's findings will be put to the test in 2017 when the European Space Agency launches the CHaracterising Satellite (CHEOPS). This will study super-Earths in more detail and should be able to tell whether some of these exotic worlds could one day be more like our own.

Explore further: Two families of comets found around nearby star Beta Pictoris

More information: Lammer, H. et al. Probing the blow-off criteria of hydrogen-rich 'super-Earths', Monthly Notices of the Royal Astronomical Society. dx.doi.org/10.1093/mnras/sts705 , 2013.

Preprint: arxiv.org/abs/1210.0793

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User comments : 15

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FrankHerbertWhines
3.7 / 5 (6) Feb 04, 2013
that would make them "gas midgets"
ahmedgnz
5 / 5 (3) Feb 04, 2013
Not habitable, yes, but if there is liquid H2O there's always a possibility these superEarths/miniNeptunes might still be life-bearing.
Budding Geologist
4 / 5 (4) Feb 04, 2013
A lot of these planets have gravities of 2-3 times that of Earth... given the prevalence of lower back problems on our own world I always doubted they'd be habitable for humans.
antialias_physorg
4.5 / 5 (8) Feb 04, 2013
given the prevalence of lower back problems on our own world I always doubted they'd be habitable for humans.

Since any transport to such a planet is on the order of hundreds to millions of years there'd be time to work something out.

Given that no extraterrestrial planet will have a 20/80 oxygen/nitrogen atmosphere (since that is purely due to the action of living organisms over bilions of years here on Earth) I think backaches would be the least of our problems.
philw1776
1.7 / 5 (3) Feb 04, 2013
The 1st picture is misleading and not to scale. As drawn, the mini Neptune is much larger than Neptune and closer to Jovian sized.
Birger
3.7 / 5 (3) Feb 04, 2013
The 1st picture is misleading and not to scale. As drawn, the mini Neptune is much larger than Neptune and closer to Jovian sized.


See "heats up the gaseous envelopes of these worlds, so that they expand up to several times the radius of each planet"
Modernmystic
3.9 / 5 (7) Feb 04, 2013
The term "Super Earth" always was a misleading hedge.

We simply haven't found many Earth like planets at all, hence the "necessity" to wiggle "Earth" in there somewhere.
typicalguy
4 / 5 (4) Feb 04, 2013
The term "Super Earth" always was a misleading hedge.

We simply haven't found many Earth like planets at all, hence the "necessity" to wiggle "Earth" in there somewhere.

Earth is too small, need better telescopes.
Torbjorn_Larsson_OM
5 / 5 (3) Feb 05, 2013
Not worried re habitability.

The amount of hydrogen would be set by the onset of core collapse, which would be differently timed in different disks. I.e. we still need to look at all these worlds as potentially habitable until we can characterize their atmospheres.

Instead, these atmospheres would be a good protection against the early strong activity of M dwarfs, the most common stars. And they look like excellent environments for early life (reducing atmospheres).

@ahmedqnz: You are using confusing terminology. Maybe you mean water worlds have no continents (but they are still surface habitable, perhaps even more so than planets with some dry land).
Torbjorn_Larsson_OM
3.5 / 5 (4) Feb 05, 2013
@ModernMystic: SuperEarths have always been defined up front as potential terrestrial types, only larger than Earth. No hedge intended or implied, obviously you can make a slightly larger terrestrial than Earth in the same way that Earth is slightly larger than Venus et cetera.

You are inserting conspiracy or fuzziness where none exist.

The question is where the cart balances over. As I noted above, this isn't enough to predict individual cases.
Modernmystic
5 / 5 (1) Feb 05, 2013
@ModernMystic: SuperEarths have always been defined up front as potential terrestrial types, only larger than Earth. No hedge intended or implied,


Then why use the word Earth? Why not say super Venus, or Super Mars? Mini Jupiter? It's a hedge to be sure.

The obvious reason is to imply habitability where no really good reason exists to be that optimistic. I'm not necessarily saying this is a bad thing. All kinds of fields do this to generate excitement and garner funding. Everyone knows science could always use more funding, I don't begrudge a little sensationalism, I begrudge the impression it gives and it's distorting the facts in a subtle way.

You are inserting conspiracy or fuzziness where none exist.


Well that's one opinion. You may be correct, but I don't think so.

The question is where the cart balances over. As I noted above, this isn't enough to predict individual cases.


If we can't predict it then just give the masses and leave the titles out....
Modernmystic
5 / 5 (1) Feb 05, 2013
The term "Super Earth" always was a misleading hedge.

We simply haven't found many Earth like planets at all, hence the "necessity" to wiggle "Earth" in there somewhere.

Earth is too small, need better telescopes.


^ VERY true.

We simply can't know yet, we're almost there I gather. It will be very exciting to ACTUALLY know how many Earthlike planets there are out there rather than seeing these huge monstrosities and trying to square pegs into round holes....
antialias_physorg
2.7 / 5 (3) Feb 05, 2013
The obvious reason is to imply habitability

I think you're interpreting WAY too much into the term. It simply means: larger than Earth but roughly in the same size bracket. No more.
That's a better way to communicate the size than giving the masses - because the average Joe doesn't know what to do with a mass-value.
Modernmystic
5 / 5 (2) Feb 05, 2013
The obvious reason is to imply habitability

I think you're interpreting WAY too much into the term. It simply means: larger than Earth but roughly in the same size bracket. No more.
That's a better way to communicate the size than giving the masses - because the average Joe doesn't know what to do with a mass-value.


The average Joe can interpret "Twice the size of the Earth", or "Four times the mass of the Earth".

This gives a very different impression to the average Joe than "Super Earth"....just ask them, they'll tell you :)
philw1776
not rated yet Feb 11, 2013
"See "heats up the gaseous envelopes of these worlds, so that they expand up to several times the radius of each planet"

They're comparing "Neptunes" whether their gas inflated envelopes are inflated by heat or not. They're not Mini when larger than Neptune. Neptune is also a gas giant with a relatively small core.