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Habitability Index Ranks Exoplanets

If we had a space-based instrument fully capable of analyzing an exoplanet’s atmosphere in place right now, where would we find our best targets? The goal, of course, is to pluck out the signature of biological activity, which means we’re looking at planets in the habitable zone of their stars, that region where liquid water can exist on the surface. Right now there aren’t many planets that fit the bill, but the day is coming when there will be hundreds, then thousands. How we optimize our search time and choose the targets with the most likely pay-off is a major issue.

Which is where a new metric called the ‘habitability index for transiting planets’ comes into play. Developed by Rory Barnes and Victoria Meadows (University of Washington), working with research assistant Nicole Evans, the index is an attempt to prioritize the selection process, looking at those exoplanets that should be at the top of our list. Says Barnes:

“Basically, we’ve devised a way to take all the observational data that are available and develop a prioritization scheme, so that as we move into a time when there are hundreds of targets available, we might be able to say, ‘OK, that’s the one we want to start with.’”

Kepler has already given us a taste of what’s ahead, with its datasets rich in planets, but when we launch TESS (Transiting Exoplanet Survey Satellite) and later PLATO (PLAnetary Transits and Oscillations of stars), we’ll be looking at a much broader field. Bear in mind that Kepler took its ‘long stare’ out along the Orion Arm, where 156,000 stars were in range to be analyzed for the lightcurves that identify planetary transits. That’s a way of using brute force methods to get a statistical reading on how common various kinds of planets are in the galaxy.

But most of the stars in the Kepler field of view are hundreds if not thousands of light years away — fewer than one percent of them are closer than 600 light years. So the starfield in Cygnus and Lyra that gave us such sensational results with Kepler will now be supplanted with the search for planets closer to home, and a TESS field of view that will cover 400 times as much sky. TESS should be a good target-finder for the James Webb Space Telescope, whose launch in 2018 will open up the possibility of studying the atmospheres of such worlds.


Image: UW astronomers Rory Barnes and Victoria Meadows of the Virtual Planetary Laboratory have created the “habitability index for transiting planets” to compare and rank exoplanets based on their likelihood of being habitable. Credit: Rory Barnes.

Barnes and Meadows presented their habitability index in a paper for The Astrophysical Journal called ‘Comparative Habitability of Transiting Exoplanets.’ The goal is to move well beyond what has thus far been the ‘first cut’ for habitable planets, the question of whether they can have liquid water on the surface. The index takes particular account of the planet’s ‘eccentricity-albedo degeneracy,’ which examines the energy reflected off its surface (albedo) and weighs it against the circularity of its orbit.

This gets into an interesting balancing act. High albedo means a great deal of light and energy being reflected back into space, so that less is available at the surface. And eccentric orbits can create intense periods of energy influx, during which the planet passes closest to its star. As the paper notes, there is an equilibrium that marks the most life-friendly zone given these factors. In the excerpt below, H refers to the habitability index number:

Our approach to comparative habitability assessments parameterizes the eccentricity-albedo degeneracy by calculating the fraction of all possible (A, e) [albedo, eccentricity] pairs that could produce a clement climate. We must also assess “rockiness” probabilistically based on inferences from the few planets with well-known masses and radii. Despite these assumptions, H can provide more insight into a planet’s potential to support life than simply comparing its orbit to that of its host star’s HZ.

Applying the index to the Kepler dataset produces some interesting results:

We ranked the known Kepler and K2 planets for habitability and found that several have larger values of H than Earth. This does not mean these planets are “more habitable” than Earth – it means that an Earth twin orbiting a solar twin that is observed by Kepler would not have the highest probability of being habitable. The best candidates have incident radiation levels, assuming circular orbits, of 60–90% that of Earth’s. These levels are about in the middle of the HZ, which is not surprising as our flux limits are derived from the models that produced the HZ. Our method is grounded in the fundamental observables and can, when applicable, fold in eccentricity constraints, and is therefore more powerful than the HZ for transiting exoplanets.

The age of the star can also be a useful data point when it comes to habitability. Here we look at the timescale on which life arises, a challenging issue because all we have to work with is what we know of life’s development on Earth. In general, the paper notes, planets in the habitable zone of F, G or K stars should see the last wave of giant impacts on their young surfaces at around 100 million years. Given that impacts likely sterilized our planet for several million years, this is an important issue, and stellar age, if known, can factor into the question of habitability.

A habitability index like this one isn’t designed for a single mission, but anticipates creating a target list we’ll want to investigate with a variety of technologies. The James Webb Space Telescope will be the first spacecraft that can perform transit transmission spectroscopy, but the paper notes that planets with high values of H could well be unobservable by JWST because of its detection capabilities. Barnes and Meadows examined four candidates for JWST spectroscopy — these were all Kepler or K2 detections — and found that even detecting water bands on these would be challenging for JWST. What TESS may identify as the best target for JWST won’t necessarily be the best target for later missions with different instrumentation.

The paper is Barnes, Meadows & Evans, “Comparative Habitability of Transiting Exoplanets,” accepted at The Astrophysical Journal (preprint). A UW press release is also available.


Comments on this entry are closed.

  • Lord October 6, 2015, 15:00

    Wonder if one wants habitability or inhabited as evidenced by an oxygen atmosphere which would put the age at plus a few billion.

  • Andrew LePage October 6, 2015, 15:16

    Kudos to Barnes, Meadows and Evans for their work on this paper, “Comparative Habitability of Transiting Exoplanets”! As most regular readers of Centauri Dreams are aware, I have written extensively here and elsewhere about the all too often overblown assessments of the potential habitability of some extrasolar planets.


    I read this new paper a few days ago and, personally, I am very pleased with what I saw. Previous metrics ranking the potential habitability of exoplanet too often used what I felt were overly optimistic assumptions and would rank planets that are more likely to be a mini-Neptune or a super-Venus as being among the most Earth-like planets known calling into question the utility of these measures. Barnes et al., on the other hand, use much more conservative assumptions and definitions of potential habitability (at times even more conservative that I typically follow) based on the latest work in the field making their metric probably the best I have seen. While even better metric are bound to be created in the years to come as we learn more about exoplanets and their environments, I am looking forward to delving more deeply into this work and using it in my own future assessments of the potential habitability of exoplanets.

  • Rangel October 6, 2015, 16:28

    Pretty interesting approach, specially the conclusion of the best candidates should get 60 to 90% Earth radiation level. It should be noted that the real temperatures on a given planet heavily depends on the atmosphere (most based on the pressure value), so we cannot take any rank as precise without these informations, anyway i always feels excited about advancements on the habitability issue.

    Personally i believe runaway greenhouse to be far more problematic for habitability, meaning i am more into middle to outer zone HZ planets than inner ones.

    Hope we can deduce atmospheric pressure on an exoplanet soon, even if the precision is low it could finally give us definitive information whatever a planet may be habitable or not.

  • ljk October 6, 2015, 16:34

    Today is the twentieth anniversary of the first exoplanet found circling a Sol type star, 51 Pegasi b:


    To quote:

    “At the time, even the most adventurous minds blithely assumed that our solar system was more or less typical, a template for all the others. 51 Peg b threw a big splash of reality in their faces. The newfound world was bizarre, a Jupiter-size world skimming the surface of its star in a blistering-fast “year” that lasted just 4.2 days. Its existence ran counter to the standard theories of how planets form and evolve. It answered one big question: Yes, other planetary systems really do exist. But it raised a thousand others.”

    “It was so strange that we decided to wait until the next observing season,” Mayor recalls. The motion of the star was far too fast and too extreme to be explained by a planet that was at all like the ones in our solar system…and yet the signal was undeniably there. After another round of observations yielded the exact same result, Mayor and Queloz wrote up a paper—only to have it rejected by one of the reviewers at Nature, who found its claims too outlandish. The editor in chief interceded, Mayor popped a bottle of champagne, and on October 6, 1995, he announced his findings at an Italian astronomy conference, to a chorus of amazement and incredulity.

  • Andrew Palfreyman October 6, 2015, 16:43

    References in the paper notable by their absence: ozone, magnetosphere, tectonics. All of which play into a true habitability index.

  • Andrew LePage October 6, 2015, 17:25

    @Andrew Palfreyman October 6, 2015 at 16:43

    “References in the paper notable by their absence: ozone, magnetosphere, tectonics. All of which play into a true habitability index.”

    This is all very true. We would want a lot more information on the atmospheric, geophysical and environmental properties to truly determine the habitability of any world. But obtaining such detailed information is currently well beyond our technology (and likely to be so for a very long time). But based on what properties we CAN measure today (orbit, mean stellar flux, radius, etc.), this index is an effective metric to gauge the relative potential habitability of an exoplanet so that they can be ranked given our current understanding of planetary habitability. This is an effective means of narrowing down the pool of potential targets for future study by new instruments and techniques as they are developed.

  • RobFlores October 6, 2015, 17:51

    So does any confirmed exoplanet exceed the habitability level of
    Titan, Enceladous, or even Mars? The Majority of these exos likely feature very dense atmospheres and/or baking surfaces.
    Even an Earth sized world close to Mars orbit would be more interesting
    that the typical lot presented up by the Kepler data thus far

  • P October 6, 2015, 19:15

    I realise this is just slightly tangental to the topic under discussion, but I thought I’d post it here anyway (Paul feel free to do with it what you will).
    Kim Stanley Robinson’s ‘Aurora’ was the topic of a recent interview on Australian ABC radio. The link is here:


    There are some interesting insights here into his current mindset about interstellar travel and planetary habitability.


  • Paul October 7, 2015, 5:31

    I have a question in my head for quite some time that I’d like to ask now: Why are the exo-planets we found so far, mostly so far away from earth? Why did we not start searching in our galactic neighbourhood? Like, why did we not begin with Alpha Centauri to search for exo-planets and went to the next closest star in ascending order?

    I hope this does not come over as an insult, I am genuinely curious about the scientific, technical or political(?) reasons.

  • Paul Gilster October 7, 2015, 7:12

    There’s nothing wrong with that question, and it has a good answer. Kepler was designed the way it was to give us a statistical look at planets in the galaxy, the idea being to take a huge sample (156,000 stars), find out how many planets we can discover around them, and extrapolate from that the larger planetary population. TESS is designed to start looking closer to home for targets we can study in more detail; so is ESA’s PLATO mission. Alpha Centauri is a tough study because the two primary stars are so close to each other, making it tricky to tease out a radial velocity signal, especially at this point in their orbits, as (from our viewpoint on Earth) they are fairly close together, but the work continues. So think of the exoplanet hunt as trying to cover the whole range — get a statistical look at the planetary population, and also find closer targets whose atmospheres we will someday study to look for signs of biology. Hope this helps a bit, but others may want to weigh in.

  • Paul October 7, 2015, 7:58

    Hey we got the same first name!

    Thanks for your answer!

  • Harry R Ray October 7, 2015, 9:41

    Andrew Le Page: Using some of YOUR arguments from previous comments you made on this website, I find some of the early results QUITE CONTENCIOUS! Case in point: Kepler 442, which is ranked HIGHER THAN EARTH on this index. With a radius of 1.3 earths, it is substantially ABOVE the currently accepted limit(some contention here, too from Sassalov, et al) of 1.2 earths to be SAFELY below the “gas midget” cut-off. Even assuming that 1.2 earths may be TOO RIGID(which I tend to agree with) My most optimistic take on this planet is it is a cool “super venus” with an atmospheric pressure at its SURFACE similar to the water pressure at the bottom of the Marianis Trench! Of course, multi-cellular life does thrive at the bottom of the Marianis Trench, but does so with absolutely no light. Even with this OPTIMISTIC apparisal of Kepler 442b, I assume that no sunlight reaches its surface. I guess it all boils down to hability being ANY KIND OF LIFE AT ALL, no mater how extreme!

  • Rafik October 7, 2015, 9:53

    @RobFlores October 6, 2015 at 17:51
    “So does any confirmed exoplanet exceed the habitability level of
    Titan, Enceladous, or even Mars? The Majority of these exos likely feature very dense atmospheres and/or baking surfaces.”

    Not sure if this has been reviewed and validated but it looks like Kepler 442b has a similar rating as Earth. So I guess that the answer is Yes.

  • ljk October 7, 2015, 10:55

    20 Years Later–a Q&A with the first Astronomer to Detect a Planet Orbiting Another Sun

    Michael Mayor and grad student Didier Queloz were the first astronomers to identify an alien world as it circled a sunlike star

    By Corey S. Powell | October 6, 2015

    Twenty years ago this month the universe became a richer, stranger and decidedly less lonely place. For centuries, visionaries ranging from Isaac Newton to Gene Roddenberry had speculated about planets orbiting other suns, analogous to the worlds of our solar system—but it was only speculation.

    Then in October 1995 Michel Mayor, an astronomer at the University of Geneva, and his graduate student Didier Queloz discovered company: the first known planet orbiting a sunlike star.

    Full interview here:


  • Andrew LePage October 7, 2015, 16:34

    @Harry R Ray October 7, 2015 at 9:41

    “With a radius of 1.3 earths, it is substantially ABOVE the currently accepted limit(some contention here, too from Sassalov, et al) of 1.2 earths to be SAFELY below the “gas midget” cut-off.”

    I am unaware of any work that states that 1.2 R_E is the cutoff between rocky planets and mini-Neptunes. Instead there is a gradual increase in the probability that a planet is a mini-Neptune with planets with radii greater than around 1.5 R_E being more likely to be a mini-Neptune than a rocky planet. The recently published work of Leslie Rogers, which is the best statistical analysis of the mass-radius relationship for exoplanets I have seen to date, also finds the transition to take place probably at ~1.5 R_E with a 95% probability that the transition happens at radii no greater than 1.6 R_E. A detailed review of that work can be found here:


    The new work by Barnes et al. reflect this latest work in the calculation of their habitability index using an admittedly ad hoc model of the probability that a planet is rocky. While a planet with a radius of 1.3 R_E has some chance of being a mini-Neptune, the work of which I am aware strongly suggests that it is much more likely to be a terrestrial planet.

  • Tom Mazanec October 7, 2015, 16:41

    Last bombardments at 100 million years?
    Wasn’t the Late Heavy Bombardment several times that age?

  • Andrew LePage October 7, 2015, 16:44

    @Paul October 7, 2015 at 5:31

    “Why did we not start searching in our galactic neighbourhood? Like, why did we not begin with Alpha Centauri to search for exo-planets and went to the next closest star in ascending order?”

    Paul Gilster has given a nice explanation but I would add that astronomers are already actively searching for planets around Alpha Centauri and almost all of the nearby stars. Details of the searches for planets around Alpha Centauri can be found in this pair of essays:



    And a year ago I posted an update on the attempts to confirm the existence of Alpha Centauri Bb here in Centauri Dreams:


    And one recent attempt to observe a transit of Alpha Centauri Bb instead found what could be the transit of a different planet:


    And this is just the tip of the iceberg for the search for planets orbiting the nearby stars. Basically, astronomers are casting a wide a net as possible using all the techniques at their disposal to find extrasolar planets where ever they might be found.

  • Paul Gilster October 7, 2015, 20:31

    I’ve also covered the Alpha Centauri hunt here in a number of posts over the years. To pick a few more recent ones:

    Centauri B: Targets and Possibilities


    Closing in on Alpha Centauri


    Proxima Centauri: Looking at the Nearest Star


    Quite a bit more in the archives.

  • Stan Erickson October 9, 2015, 13:12

    Perhaps habitability should be fractured into several parts to help those scientists involved with clarifying some criteria for them. May I suggest: 1) able to originate life; 2) able to have life evolve from chemotrophs to photosynthetic organisms; 3) able to evolve life on land; 4) able to evolve intelligent life. A different set of classes relates to the ability to support life that has already evolved, such as ours, and can arrive in a spaceship. Those categories might be 1) able to support life on the surface as is ( a new Earth); 2) able to support life after some terraforming; 3) able to support life in hermetic vessels; 4) able to have robotic presence.