Researchers Create 'Habitability Index' For Exoplanets

hypnosec writes: The Kepler Space Telescope has allowed astronomers to detect and catalog thousands of exoplanets and exoplanet candidates. With more powerful telescopes like the James Webb Space Telescope scheduled for launch, scientists will be able to check if any of these exoplanets are habitable. But these space telescopes are expensive to create, and access time is coveted. This means simply pointing telescopes to random exoplanets isn't a practical proposition. That's why researchers have created what they call a "habitability index for transiting planets," with which astronomers will be able to prioritize the use of space telescopes for finding habitable planets. Their paper is available at the arXiv.

Comments

We already had one

[russotto]

The most habitable worlds, of course, are class M.

Re:We already had one

[dgatwood]

The most habitable worlds, of course, are class M.

Well, you should at least find plenty of Roddenberries.

Re:We already had one

[Grishnakh]

Yes, they're fictional, but it's a good start, since after all it proves that modern-day researchers weren't the first people to think of classifying planets by their habitability characteristics. Also, the simple "class [letter]" scheme is easy to remember and use; I sure hope they don't come up with some arcane, complicated system instead. Finally, they should definitely use "Class M" to refer to Earth-like planets simply to pay homage to Star Trek. Everyone and his brother knows what a "Class M" planet is, as long as they watched some Star Trek within the last 50 years.

It looks like you got your classes from Star Trek too, as seen here, but with some differences. I'm not sure where you got Class Q or I. The system probably does need a little revision though. Class H's "generally uninhabitable" doesn't tell you why. The Class P (see appendices) for icy planets is a good example. Class N for "sulphuric" really isn't sufficient; Venus is more like the Class Y "demon planet" except there's no dilithium-based biomimetic lifeforms, but the fact that Venus is so hot is important it needs to be classified that way. If a planet is too cold or too hot to live on, that's an important factor for humans. Same if there's no atmosphere. A planet (or moon) that's not too warm or hot but has no atmosphere can still be inhabited using domes or other sealed habitats, so that should be a class by itself. Mercury probably wouldn't fit there however, because it's much too hot. But it's hot in a different way than Venus, so they should have different classifications (hot because it's too close to the star, vs. hot because it has a thick atmosphere and runaway greenhouse effect). Finally, moons and planets should be classified together. The orbital path doesn't really matter (except insofar as it affects the climate/temperature). There could very well be Earth-like moons out there somewhere, so those should be Class M (like the moon in "Avatar").

So here's my proposal which borrows from ST:
Class M - Earth-like, small, rocky, oxygenated atmosphere, right temperature
Class D - small, rocky, little to no atmosphere, right temperature, inhabitable with sealed habitats (e.g. Mars)
Class J - gas giant (any size; this may be expanded later after we explore more star systems and decide we need to classify them further)
Class E - small, rocky, little to no atmosphere, too cold (e.g. Pluto)
Class F - small, rocky, little to no atmosphere, too hot (e.g. Mercury)
Class G - small, thick atmosphere, too hot (e.g. Venus)
Class A - very very small, not spherical (e.g. moons of Mars, captured asteroids)
Class B - very small, spherical but extremely low gravity (e.g. Sedna, Ceres, Pluto, dwarf planets in general)

I'm probably missing something here, perhaps planets with only liquid surfaces. I avoided calling Venus "Class N" because it sounds too much like "Class M".

Re:We already had one

[Anonymous Coward]

Class H's "generally uninhabitable" doesn't tell you why.

It's typically because there is a giant blob of an amorphic entity bent on making doppelgangers of anyone who steps foot on them. Also the atmospheres are typically highly corrosive. The "H" in "Class H" is short for "Hell."

Re:My question is ...

[Rei]

I think it's silly in the regards that we have precisely one datapoint about the sort of environments in which life may exist, which is pretty terrible in terms of making any sort of definitive statement. I'd much rather they keep their options open, check out a wide range of environments, and just look for signs of "things that are hard to explain", whatever they may be. "Hmm, this body has both a strong oxidizer and a strong reducing agent in its atmosphere - how is that happening?"

I'm not saying "check planets in random order" or anything of that nature. Just that I don't think it's critical to obsess over being sure to examine them in order of "earthishness" from highest to lowest. We need to be looking at a diversity of worlds.

Heck, we don't even know whether the surface of a body is the best place to look, most life in the universe might be in sub-crustal layers for all we know. Certainly would partially help explain the Fermi paradox, if it were such that we rare "surface dwellers" have a far easier route to the cosmos than something that needs to be under gigapascals of pressure to survive and whose radiating transmissions, if any, would be blocked by their planet's crust.

Re:My question is ...

[Rei]

I have no clue where you're coming from. You rightly point out that life takes energy, but then proceed to consider internal sources of energy as worthless, when in reality in the universe far more things are exposed to internal energy than external. And radioactive decay-driven energy sources are only one. For example, Encelaldus's heat seems to be driven by the serpentization of rock, which also releases hydrogen, a potential food source to microorganisms. There are numerous chemical means which can release vast amounts of energy - yes, nuclear energy is many orders of magnitude more dense, but non-radioactive elements are also orders of magnitude more common.

Anywhere that there is heat and fluids (or solids that can undergo solid-state convection) can experience that heat being turned into harvestable forms of chemical energy, because chemical equilibriums are different at different temperatures. For example, at STP conditions, N2 + O2 is favorable, while at high temperatures NO2 is more favorable. N2 + O2 that goes to higher temperatures and forms NO2, which then comes back down to the lower atmosphere, is bringing a source of chemical energy with it.

Since heat differentials can and will be readily converted to chemical energy wherever it's associated with convection of any variety, then any source of heat is a fuel for life - and heat most definitely doesn't only come from nuclear decay - or chemical reactions. It comes also from the rebalancing of layers to a lower gravitational equipotential. It comes from impacts. It comes from tidal heating. It comes from thermal cycling in elongated orbits. It comes from mass loss due to solar wind exposure. There's a vast range of potential heating sources in the universe that can create heat differentials. And heat differentials make exploitable chemical reactions.

You make blind assertions that "these environments wouldn't be likely because of their composition". What do you know about this? You have a sample size of one of chemical processes that have created life. We can't even see deep into our own world to see what other alternatives might exist at higher pressures, let alone in other worlds. Heck, underground doesn't even mean particularly high pressures. Dwarf planets can have Earth-surface pressures at hundreds of meters or even kilometers depth. And life on Earth exists fine in the deep sea, wherever there's energy to support it, where pressures are at over 1000 atmospheres

Deep environments might prove even more prone to organic chemistry. In general, pressure is usually associated with faster reaction rates. You also often have more complex arrangements of possible chemical phases for each compound at higher pressures than with lower pressures. Water for example over its possible temperature range at a particular depth might have 3-5 potential ice phases, a liquid phase, a supercritical fluid phase, and a gas phase. This leads to a much greater range of possibilities for reactions to potentially exploit, because each chemical in each of its phases has the potential it interact with each other chemical in each of its other phases, or in the case of non-metastable forms, at least many of its other phases.

Common theories for the origin of life on Earth usually assume that it wasn't the sun that powered the first forms of life, even though that's the most convenient source of energy on our planet. Photosynthesis is much more complicated than most forms of chemosynthesis. Environments like black smokers, volcanic pools or acidic waters within deep iron-rich minerals seem like far more likely candidates.

Intelligence evolving within creatures that live in liquids? Oh, we've never seen that before! ;) Except, of course, for the fact that the second-most intelligent category of mammals are aquatic (cetaceans), and the most intelligent invertebrates (mollusks) live there too. Rather, the oceans tend to be highly competitive environments, and thus good breeding grounds for intelligence.