New theoretical models aid the search for Earth-like planets

Nov 14, 2013
This artist's conception illustrates Kepler-22b, a planet discovered by the Kepler space telescope. The relation between the planetary mass and radius will determine whether this is an Earth-like planet. Credit: NASA/Ames/JPL-Caltech

Researchers from Bern have developed a method to simplify the search for Earth-like planets: By using new theoretical models they rule out the possibility of Earth-like conditions, and therefore life, on certain planets outside our solar system – and limit their search by doing so.

Currently extensive observational programmes are being developed all over the world, with the aim to detect outside our solar system that are able to accommodate life – a sheer impossible task. "The question whether so-called exoplanets are habitable or not is difficult to answer, as we do not know all the necessary conditions a planet has to fulfil in order to be habitable", said Yann Alibert of the Center for Space and Habitability (CSH) at the University of Bern.

This is why the Bernese scientist chose an alternative approach for his study, which is published in the journal Astronomy & Astrophysics: Based on the mass and radius of a planet Yann Alibert was able to determine criteria that exclude the possibility of life as we know it. The data required first, planetary mass, is for example provided by the HARPS-spectrograph in Chile, which was developed by the University of Geneva and Bern in cooperation with further partners. From 2017 the space telescope "CHEOPS", developed and built under the supervision of the ESA and the CSH, will be used to accurately determine the radius of certain planets, the second required data. Thanks to Yann Alibert's method, one is able to deduce whether a planet is unhabitable from the data provided by HARPS and CHEOPS. "This theoretical model will help astronomers concentrate on promising candidates in their search for Earth-like planets", says Alibert.

No life without a Carbon cycle or water in liquid form

Two conditions, without which life is not possible, form the foundation of the : Water in liquid form and a so-called Carbon cycle must be found on the exoplanet. The Carbon cycle is a geological process that regulates the CO2-level in the atmosphere and with that, the temperature of the planet's surface: In the ocean, CO2, in its dissolved form, undergoes a chemical reaction and is then transported into the Earth mantle. Because of the high temperature in the inner parts of the Earth mantle, the CO2 is released back into the atmosphere during volcanic eruptions.

Exotic ice makes planets hostile

If, however, a planet with a given mass has a very large radius, the density will be very low. Consequently there will be no Carbon cycle or liquid water on that planet. The reason for this is that low density is an indicator for a lot of gas and/or water. If a planet consists of a lot of gas, the atmospheric pressure on the surface may be so high that water is not able to keep its liquid form.

If the planet is covered by an immense amount of water, the pressure at the bottom of the ocean will increase to such an extent that water occurs in the form of "Ice VII", which does not exist on Earth. "Ice VII" has such a high density so that it settles on the ocean floor. There, it forms a barrier between the rocks on the ocean floor and the water above – preventing the Carbon cycle.

"Our study shows that a planet, that consists of a lot of gas or water, is not habitable", explained Yann Alibert.

Most "Super Earths" are not habitable

The largest radius, at which a Carbon cycle and liquid can occur, depends on the planetary mass: A planet with the same mass as Earth can have, at the maximum, a radius 1.7 times the Earth radius, including the gas and hydrosphere. A "Super Earth ", 12 times more massive than Earth, can have a radius 2.2 times the Earth radius. However, according to Alibert, mainly larger exoplanets have been discovered up until now. In the near future, smaller and more promising planets will be targeted, thanks, in particular, to the CHEOPS's high sensitivity.

Explore further: An old-looking galaxy in a young universe

More information: Yann Alibert: On the radius of habitable planets, Astronomy & Astrophysics, November 2013, Available: arXiv:1311.3039v1

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User comments : 7

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1 / 5 (10) Nov 14, 2013
I pointed out these exact same things several times over the past 3 years or so, and usually get a "1" response from the Forum Mafia.

You guys have so much egg on your faces now.
Nov 14, 2013
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2.3 / 5 (3) Nov 15, 2013
Interesting but not very useful right now. We've found only 1 or 2 superearths in the goldilocks zone so far.
2.1 / 5 (7) Nov 15, 2013
I pointed out these exact same things several times over the past 3 years or so, and usually get a "1" response from the Forum Mafia.

You guys have so much egg on your faces now.

Well here, let me help you that Skippy.

Anyone who is familiar with my posts on Exoplanet topics knows this. I was just making a reminder.

I made the EXACT same arguments regarding exotic ices at the bottom of oceans as well.

Though his mass calculations are a bit different than what I estimated, I was only basing my estimates on macroscopic, multi-cellular life. I figured the limits for that are somewhere around 8 Earth masses. His calculation is for all Carbon-based cellular life.

Here's one, but not the one I had in mind:

My third post on the thread, Jackass:

This was not my first time pointing this out, but It proves I'm not making it up.
Nov 15, 2013
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1 / 5 (4) Nov 15, 2013
So many Earth-Like planets in our galaxy and no sign of extraterrestrial intelligence detected yet by our radio telescopes. But when we remember the dangers, rare at our time scale but deadly real, which exist in our galaxy (asteroids, comets, neutron stars, black holes, supernovae), we might envision the possibility that any advanced civilization would decide at some moment in its evolution to build its own artificial worlds, fully controllable and perfectly autonomous in cosmos, capable to create artificial gravity, relying on self-sustaining closed ecosystems and on extremely powerful long-lasting energy sources like nuclear fusion. Such gigantic artificial worlds would not be related to a solar system and they could perfectly function in the vast space between galaxies where there is no danger of cosmic collisions, black holes, supernovae, etc. So the greatest cosmic boulevards could be outside the galaxies, in intergalactic space.
2 / 5 (4) Nov 15, 2013
"Our study shows that a planet, that consists of a lot of gas or water, is not habitable", explained Yann Alibert.
Not habitable, or life cannot evolve there? There is a difference.
not rated yet Nov 16, 2013
Creationists, whether trolling with rejected Rare Earth models or spouting the usual creationist ideas forthright, are hilarious. They only help make deconverts from religious magic ideas, see Dawkins's Convert's Corner.

It is surprising that the rare systems that end up with 30 % carbon by mass or more can't have liquid water no matter what. But we have been fairly certain carbon worlds were inhabitable anyway, and the same with water worlds - little - to no rocks, so no minerals for key enzymes. And the same goes for terrestrials with more than 2 Earth radii, which (as the paper points out) seems to be volatile rich miniNeptunes rather than habitable superEarths.

not rated yet Nov 16, 2013

So nothing that bars astrobiology here, despite unsupportable claims to the contrary.

The main contribution here and in similar recent works is the systematics, and that we now know that terrestrials with 1-2 Earth radii and less than 3 % carbon have the most productive biospheres. They can have a full plate tectonic supported carbon cycle.

Those with 3 - 30 % carbon are still habitable (have liquid water), but will likely not see an oxygenated atmosphere. So multicellulars, but not complex multicellulars.

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