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Habitable Zone Planets and Kepler

A habitable zone can be defined in many ways, but for our immediate purposes, defining it with reference to liquid water on a planetary surface makes sense. Sure, we believe that life could exist beneath the surface on places like Europa, where surface water is out of the question, but the key issue is this: Are there atmospheric features that we could use to make the call on habitability? It’s an important issue because with our current and near-future technology, this is how we can plan to investigate life on planets around other stars. We can study exoplanetary atmospheres already and we’re getting better, but we can’t drill through exoplanetary ice.

A new paper from Lisa Kaltenegger and Dimitri Sasselov (Harvard Smithsonian Center for Astrophysics) gets into these questions by looking at how to evaluate habitability, studying different kinds of planetary atmospheres and developing model calculations. The intent is to apply these ideas to the habitable planet candidates, 54 in number, so far produced by the Kepler mission, but the work generalizes to planetary candidates from any transit searches. The smallest size objects in the current Kepler sample are clearly the most interesting, because they could have the kind of atmospheres we see from Venus through the Earth to Mars in our own system, a range of chemistries that points out the limits of this kind of habitable zone.

A habitable zone can be conceived as a ring — an annulus — around a star, and the key points are these:

… the inner edge of the HZ is defined as the location where the entire water reservoir can be vaporized by runaway greenhouse conditions, followed by the photo-dissociation of water vapor and subsequent escape of free hydrogen into space. The outer boundary is defined as the distance from the star where the maximum greenhouse effect fails to keep CO2 from condensing permanently, leading to runaway glaciation.

Not all planets in a habitable zone will be habitable, of course, but we can say many things about the potential for habitability. The width and distance of a habitable zone around a particular star depend on a number of parameters that we can hope to examine from Earth or space-based missions as our technology improves:

  • Incident stellar flux. Here we’re looking at the luminosity of the host star, its spectral energy distribution, and the eccentricity of the planetary orbit.
  • Planetary albedo, the reflecting power of the planetary surface.
  • The concentration of greenhouse gases in the planet’s atmosphere.
  • The energy distribution in the planet’s atmosphere.

The authors look at three models of Earth like planets, from planets with high concentrations of carbon dioxide in their atmospheres to those with atmospheres more or less like the Earth today and those with high values of water vapor, modeling these for stars with effective temperatures between 3700 K and 7200 K. The models thus represent planets on the outer edge of a habitable zone, in the middle of it, and on the inner edge. Examined in this way through a straightforward equation developed by the authors to measure potential habitability, many of the 54 planetary candidates thought to be in the habitable zone in the Kepler sample turn out to be outside of it because their temperatures are too high. 27 of the Kepler candidates show temperatures in the habitable range, three of them being candidates with radii smaller than two Earth radii.

Cloud cover is an intriguing variable. In fact, the authors point out that depending on cloud cover, the outer edge of the habitable zone in our own Solar System varies from 1.67 AU (cloud-free) to 1.95 AU and 2.4 AU (50% and 100% cloud cover respectively). Assuming cloud cover of 0%, 50% and 100%, we get three sets of results:

Applying our analysis to the whole Kepler planetary sample of 1235 transiting planetary candidates, assuming the maximum Earth-like Bond albedo for rocky planet atmospheres… results in 12, 27, 67 planetary candidates with Teq [equilibrium temperature] smaller than the water loss limit (Tsurf = 373K) for 0%, 50% and 100% clouds respectively, and 18, 43, 76 planetary candidates with temperatures lower than the runaway greenhouse limit respectively. Among those are 2, 3, 6 as well as 3, 4, 6 planets respectively, that have radii below 2 Earth radii consistent with rocky planets (KOI1026.01, 854.01, 701.03, 268.01, 326.01, 70.03).

We can use transit timing variations to calculate the density of rocky planet candidates occurring in multiple planet systems, but remember that the fact that a planet is rocky does not necessarily make it habitable, depending on the local abundance of water and other materials. That means a good terrestrial planet candidate has to have its atmosphere fully examined for signs of life, and here Kepler will not be enough. Its target stars are 500 to 1000 parsecs from us, making its findings extremely helpful statistically but less useful than a future targeted mission at finding small planets orbiting stars closer to the Sun whose atmospheres we can analyze.

The paper is Kaltenegger and Sasselov, “Exploring the Habitable Zone for Kepler planetary candidates,” submitted to Astrophysical Journal Letters (preprint).


Comments on this entry are closed.

  • Alex Tolley May 9, 2011, 13:17

    “The outer boundary is defined as the distance from the star where the maximum greenhouse effect fails to keep CO2 from condensing permanently, leading to runaway glaciation.”

    Is it possible that methane out gassing could extend the habitability zone beyond this limit? This out gassing need only be very slow, providing enough GH effect to keep the CO2 from freezing.

    We suspect our own planet was ice covered in ice at least temporarily before 650mya, and that methane from volcanoes could have contributed to a global warming. Could this mechanism be enough for worlds where CO2 could freeze out, but not CH4, extending the habitability zone outwards?

  • andy May 9, 2011, 14:32

    To be honest, I was a bit surprised to see the criteria used when the Kepler people released all the candidates and claimed a bunch of them were habitable: they essentially used an effective temperature in the range 0 to 100 degrees C, then decreased the lower boundary to account for greenhouse warming. This is despite the fairly recent and very public case of Gliese 581c which was similarly announced as potentially habitable based on an effective temperature in the 0 to 100 degrees C range. (In fact if you model the planets as uniformly-radiating spheres with albedo 0.3 to match the Earth, the temperatures of Earth and Venus come out as 255 and 300 K respectively, that latter figure being deceptively temperate-looking!)

    This analysis seems to be more realistic in the way they treat the habitable zone. Then again, given we are finding that many super-Earths have thick, potentially hydrogen-rich atmospheres, the outer boundary may lie much further away from the star than the traditional analysis suggests.

  • kurt9 May 9, 2011, 15:55

    …we are finding that many super-Earths have thick, potentially hydrogen-rich atmospheres…

    Which means that Kepler has found many Neptunes that happen to be in the HZ’s. Not many “Earth-like” worlds have actually been found. Kepler has found only three by the definition of this paper, and even those are a stretch. The paper defines an “Earth-like” world as one that has a diameter of less than 2 times the Earth.

  • Ronald May 10, 2011, 5:29

    My impressio0n from various authors on the criteria and definition of HZ is that its outer boundary is rather fuzzy, also largely depending on planetary (atmospheric) conditions and potentially extending far out (from 1.2 to over 1.5 AU in our solar system), but that its inner boundary is quite sharply delimited, depending mainly on stellar characteristics (about 0.95 AU in our own case).
    Remarkably (or not?) my impression from literature through the years is something similar for stellar luminosity and spectral type: that the ‘cooler’ end of ‘Habitable stars’ possibly extends quite far into the later spectral types (at least K2/3, …), but that at the hotter end the limit is sharper and nearer to our sun: G0, F9? This, because at the hot side of the spectrum the star first emits lots of very aggressive radiation, then evolves much quicker (i.e. getting hotter), the HZ moves outward quickly and the window of opportunity for (higher) life to develop simply becomes too short.

  • Stephen Ashworth May 10, 2011, 8:39

    I would suggest we drop references to the misleadingly named Habitable Zone, and refer instead to the Surface Water Zone or SWZ.

    Broadly speaking, there are four known domains of life, which may be called: microbiota (single-celled creatures only), thalassobiota (oceanic multi-cellular life), geobiota (terrestrial multi-celled life), and technobiota (technological life). (The last of these has not yet been observed, but our own species is so clearly a halfway stage between the geo and techno forms that the possible existence of the latter cannot be in any reasonable doubt.) Clearly these domains are related in a hierarchical fashion, and the presence of any one domain implies also the presence of those lower in the hierarchy.

    For three out of these four, the “habitable zone” concept is not relevant, therefore the term is misleading. Only a geobiota is dependent upon its habitation in a Surface Water Zone.

    Oxford, UK

  • andy May 10, 2011, 16:39

    Which means that Kepler has found many Neptunes that happen to be in the HZ’s. Not many “Earth-like” worlds have actually been found.

    Seems that the division between the terrestrial-type planets and the mini-Neptunes occurs at lower masses than was once thought, though to be fair some of these “Neptunes” might be outgassed rocky planets – it doesn’t help that the Kepler candidate host stars are in general too faint to do the follow-up analysis to determine this. The 2-Earth radii criterion is if I remember rightly based on the size of a 10-Earth mass rocky planet, something that may only be relevant for the very hottest planets where atmospheric loss would occur over timescales less than the age of the system (e.g. Kepler-10b).

    Still wondering where the analogues of the inner solar system are, at present the PSR B1257+12 system still seems to be the best match. Makes me wonder what the inner solar system and the pulsar planets (which presumably formed from a disc created from supernova fallback material) have in common that they ended up in a fairly similar configuration.

  • Ronald May 12, 2011, 7:00

    Stephen Ashworth: “I would suggest we drop references to the misleadingly named Habitable Zone, and refer instead to the Surface Water Zone or SWZ”.

    In that case I would prefer Liquid Water Zone, LWZ, because that is how it was defined, on the basis of possible presence of *liquid* (surface) water.

  • Astronist May 13, 2011, 19:32

    Ronald, thanks. Obviously, I meant water as distinct from ice or steam, rather than dihydrogen monoxide the chemical substance. But its location on the surface is crucial; Europa has water (as well as ice), but is not in the zone which Paul is calling the habitable zone, above.

    Oxford, UK

  • ljk May 14, 2011, 17:53

    UC Berkeley SETI survey focuses on Kepler’s top Earth-like planets

    By Robert Sanders, Media Relations | May 13, 2011

    BERKELEY — Now that NASA’s Kepler space telescope has identified 1,235 possible planets around stars in our galaxy, astronomers at the University of California, Berkeley, are aiming a radio telescope at the most Earth-like of these worlds to see if they can detect signals from an advanced civilization.

    The search began on Saturday, May 8, when the Robert C. Byrd Green Bank Telescope – the largest steerable radio telescope in the world – dedicated an hour to eight stars with possible planets. Once UC Berkeley astronomers acquire 24 hours of data on a total of 86 Earth-like planets, they’ll initiate a coarse analysis and then, in about two months, ask an estimated 1 million SETI@home users to conduct a more detailed analysis on their home computers.

    “It’s not absolutely certain that all of these stars have habitable planetary systems, but they’re very good places to look for ET,” said UC Berkeley graduate student Andrew Siemion.

    Full article here: