The Habitable Exoplanets Catalog is Now Online!


Anyone who has an interest in exoplanets probably knows about the various online catalogs that have become available in recent years, such as The Extrasolar Planets Encyclopaedia for example, providing up-to-date information and statistics on the rapidly growing number of worlds being discovered orbiting other stars. So far, these have been listings of all known exoplanets, both candidates and confirmed. But now there is a new catalog published by the Planetary Habitability Laboratory (a project of the University of Puerto Rico at Arecibo), which focuses exclusively on those planets which have been determined to be potentially habitable. The Habitable Exoplanets Catalog is a database which will serve as a key resource for scientists and educators as well as the general public.

As of right now, there are two confirmed planets and fourteen candidates listed, but those numbers are expected to grow over the coming months and years as more candidates are found and more of those candidates are confirmed. There is even a listing of habitable moons, whose existence have been inferred from the data, although none have been observed yet (finding exoplanets is challenging enough, but exomoons even more so!).

According to Abel Méndez, Director of the PHL and principal investigator, “One important outcome of these rankings is the ability to compare exoplanets from best to worst candidates for life.” He adds: “New observations with ground and orbital observatories will discover thousands of exoplanets in the coming years. We expect that the analyses contained in our catalog will help to identify, organize, and compare the life potential of these discoveries.”

The big question of course is whether any habitable planets are actually inhabited, two different things. To help answer that, it will be necessary to further analyze the atmospheres and surfaces of those planets, looking for any indication of possible biosignatures such as oxygen or methane. Kepler can’t do that directly, but subsequent telescopes such as the Terrestrial Planet Finder (TPF) will be able to, and provide a more accurate assessment of their physical composition, climate, etc.

Not long ago it wasn’t known if there even were any planets orbiting other stars; now we’re finding them by the thousands and soon we’ll be able to distinguish their unique physical characteristics and have a better idea of how many habitable worlds are out there – exciting times.

37 Replies to “The Habitable Exoplanets Catalog is Now Online!”

  1. This is a great list, but with the heading “potential habitable worlds” as opposed to “potential habitable planets”, that info graphic should really include moons like Titan, Europa, and Enceladus.

  2. @Patrick Perhaps they had good reasons. Let’s see:

    Titan: Surface temperature = -180 C
    Europa: Surface temperature = -150 C
    Enceladus: Surface temperature = -130 C

    None of those bodies are in the habitable zone, and there is definitely no liquid surface water on any of them.

    1. Both Europa and Enceladus are believed to have oceans of liquid water below the surface. It is even now thought that Titan might have a subsurface liquid water-ammonia ocean. As we now know from studies on Earth, water trapped deep below surface layers of ice can indeed be inhabited. This makes them potentially habitable despite the fact that the surface conditions for all three are still extremely hostile.

      1. Given the limited information, I would speculate that any of the above moons are more likely to be inhabited than the known exoplanets.

      2. I don’t think anyone was trying to make that leap of logic lol. I don’t want to trespass on the thread any further though

      3. I suspect that Corrie means habitable by humans, and Eric means habitable by microbes. So really you’re in agreement.

      4. You’re clearly thinking in terms of human habitation, whereas this article and the rest of us are thinking in terms of the traditional astrobiology definition of habitable as a possible location of any sort of life.

        And even that said, it’s not hard to imagine humans finding a way to live as comfortably on Enceladus or Europa as they could on Mars.

    2. This is a catalog for what I call “surface habitability”. Planets like Mars or Europa have different biosphere capabilities, and while Mars type planets with potential crustal habitability can be approximately assessed by a proxy of surface habitability measures Europa type planets can not.

  3. Virgin galactic holidays pmsl… On a serious note, I’m very happy that the James Webb is still on 😀

  4. @EricEdwin Perhaps Venus is in the habitable zone, but Venus is not even remotely habitable. We know this for a fact. We cannot put Venus on a list of potentially habitable planets.

    Mars is still potentially habitable. We’ve yet to determine one way or another.

    1. It was a jest at your response to Pat. As we learn more, the term “habitable zone” will fall out of use.

  5. It’s really hard to tell what they’re getting at with that graphic. It sure seems like they’re implying that Mercury, Venus, Earth and Mars are all ‘habitable’, yet they’re probably just including them for scale. Sort of misleading, since only Mars is traditionally thought of as potentially habitable out of these (and I guess I’ll give you Earth!).

    And @Corrie- your assessment goes against the thinking of most/all astrobiologists. Sure, they aren’t in the traditional ‘habitable’ zone, but they are certainly ‘potentially habitable’, due largely to tidal heating, and my sense of the field is that most astrobiologists are more excited about the potential for life on these moons than they are about the potential of life on Mars. As to your argument about liquid water on the surface, well two almost certainly have sub-surface liquid water, and Titan has liquid on the surface and even entire liquid-based weather systems, albeit of methane.

    This graphic is titled “potential habitable worlds in the universe”, so I’d say the moons should be included.

    1. As I understand it, Mercury, Venus, Earth and Mars are included just for scale, as it says in the text.

      1. Abel Méndez confirmed to me that they are just included in the graphic for size comparison, nothing else.

      2. Cool, that’s the conclusion I came to after looking at it more than once, but good to have it cleared up.

    2. A planet that is habitable for humans needs to have liquid surface water, not liquid surface methane.

      You have no idea what you’re talking about.

      1. Planetary habitability refers to life in general, not just human life. So if any Jovian or Saturnian moons sustain life, they are very much habitable.

        I suspect that is an honest mistake though, so I won’t make the arrogant claim that you have no idea what you’re talking about.

      2. We know of no life that can live in those conditions. This catalogue is for life we know about, not imaginary life.

      3. Yes, we do. The deepest parts of our oceans are inhabited, cut off completely from sunlight. Have you not heard of black smokers (hydrothermal vents) on the ocean bottom which sustain a wide variety of both macrobial and microbial life? A new species of shrimp I think it was was also recently found living in waters permanently covered by ice, similar to Europa or Enceladus.

  6. The website is interesting to go to. The planets estimated to be most Earth-like have short orbital periods. As such I suspect these planets are around M-class stars and are tidally locked. I then suspect this might take them down a notch or two on the ESI.


    1. If you look on the ESI definition, it doesn’t care about tidal lock.

      Nor, I would argue, would biospheres.

      If you are concerned about temperatures, the habitability zone will only shift around with the lock. If you are concerned about atmospheric protection from a magnetic field, again habitability will only shift around with the initial atmosphere density.

      Essentially the concern about tidal lock is a zero sum game.

      1. The tidal lock issue is uncertain. I think there would have to be a lot of climate modeling done on extra-solar planets with a range of parameters adjusted. The parameters would include atmospheric pressure, chemical composition of the atmosphere, water content, and of course celestial input such as stellar energy, orbital data, and so forth. The problem with tidal locking as I see it is the planet could have extreme temperature differences, a hyper-torrid zone facing the star and a cryrogenic zone facing outwards and a band in between. So thermal energy difference dQ = dE + dU plays a big role and the dE is going to involve pdV, where p is pressure. Is suspect there would be violent winds and if the atompheric pressure average is high enough these might equilibrate temperatures so the planet is a venusian type of world.


  7. A planet catalogue??!?!? All this time I’ve been shopping from the Sears Catalogue. I’ll take the big one. How much? Can you ship before Christmas? My son will love it!!!

  8. Every time I read an article like this I actually shed tears of joy, remembering the countless hours as a child I spent fantasizing about this very topic. I LOVE SCIENCE! …lol!

  9. Not only do I notice some spelling and grammatical errors (which is expected) I also notice quite a few inconsistencies in the data and classifications. I think they are all due to new data and changes in the metrics. Some of the changes are noted, other are not and are confusing to me, not trying to be too nit-picky but some clarification would be appreciated.

    Under “A Mass Classification for both Solar and Extrasolar Planets” it states: “PSR 1257+12 b is the only example of a exoplanet in the category mercurian and was discovered by the Arecibo Observatory, there are no subterrans.” but then the table of planets under “A New Online Database of Habitable Worlds.” it lists 15 subterrans. Now after finding older tables I realized that the information on the first page was earlier and is outdated but page had been recently edited and that note had not been changed. Okay, okay, I still might be nit-picking there but then there is the issue of the classification of Mars which is truly baffling me. “A Thermal Planetary Habitability Classification for Exoplanets” lists Mars’ habitability classification as a hypopsychroplanet but all other references I saw on the site list it as a psychroplanet. What gives?

    1. There was an error in the Mars temperature which made the software assign it to the psychroplanet class instead of the correct hypopsychroplanet class. Thanks for spotting it. That is now corrected. There are no subterrans in the confirmed exoplanets so far as stated. The press release table included both confirmed and unconfirmed exoplanets Kepler candidates, and there are subterrans in the Kepler candidates. We will be revising the text and adding more things in the following weeks.

      1. Okay, yeah I saw that on your twitter. Despite some confusion on things like that I really enjoy the PHL website, I have it bookmarked. Thanks for clarifying the note about the subterrans. Overall I think your planet classification scheme is excellent and should be easy to understand and modify with future discoveries. I wouldn’t be surprised to see it eventually become ‘THE’ scientific standard of exoplanet classification.

  10. As I have mentioned several times on UT, I am a great fan of the Standard Primary Habitability index since it comprises the two dominant factors (temperature and water, aka liquid water) and matches nicely the Net Primary Productivity index of ecosystems.

    The Earth analog indexes are valuable but take the backseat together with other secondary characteristics in establishing potential for biospheres. Hence I would quibble with the decision to not list the potential superEarths of radius larger than 2.

    However, this is a laudable effort!

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