Astronomers devise 'habitability index' to guide future search

October 6, 2015 by Peter Kelley, University of Washington
The James Webb Space Telescope, a large infrared telescope with a 6.5-meter primary mirror, is scheduled to be launched on an Ariane 5 rocket from French Guiana in October of 2018 and will be the premier NASA observatory of the next decade, serving thousands of astronomers around the world. UW astronomers have created a “habitability index for transiting planets” to help guide the ongoing search for life beyond Earth. Credit: NASA

Powerful telescopes are coming soon. Where exactly shall we point them?

Astronomers with the University of Washington's Virtual Planetary Laboratory have created a way to compare and rank exoplanets to help prioritize which of the thousands discovered warrant close inspection in the search for life beyond Earth.

The new metric, called the "habitability index for transiting planets," is introduced in a paper accepted for publication in the Astrophysical Journal by UW astronomy professors Rory Barnes and Victoria Meadows, with research assistant and co-author Nicole Evans.

"Basically, we've devised a way to take all the observational data that are available and develop a prioritization scheme," said Barnes, "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.'"

The Kepler Space Telescope has enabled astronomers to detect thousands of exoplanets, those beyond our solar system—far more than can be investigated one by one. The James Webb Space Telescope, set for launch in 2018, will be the first able to actually measure the atmospheric composition of a rocky, possibly Earthlike planet far off in space, and so vastly enhance the search for life.

Astronomers detect some planets when the worlds "transit" or pass in front of their host star, thus blocking some of the light. The Transiting Exoplanet Survey Satellite, or TESS, is scheduled to launch in 2017 and will find many more worlds in this way. But it's the Webb telescope and its "transit transmission spectroscopy" that will really be able to study planets closely to hunt for life.

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

But access to such telescopes is expensive and the work is methodical and time-consuming. The Virtual Planetary Laboratory's index is a tool to help fellow astronomers decide which worlds might have the better chance of hosting life, and so are worthy of focusing limited resources on.

Traditionally, astronomers have focused the search by looking for planets in their star's "habitable zone"—more informally called the "Goldilocks zone"—which is the swath of space that's "just right" to allow an orbiting Earth-like planet to have liquid water on its surface, perhaps giving life a chance. But so far that has been just a sort of binary designation, indicating only whether a planet is, or is not, within that area considered right for life.

"That was a great first step, but it doesn't make any distinctions within the habitable zone," Barnes said. "Now it's as if Goldilocks has hundreds of bowls of porridge to choose from."

The new index is more nuanced, producing a continuum of values that astronomers can punch into a Virtual Planetary Laboratory Web form to arrive at the single-number habitability index, representing the probability that a planet can maintain liquid water at its surface.

In creating the index, the researchers factored in estimates of a planet's rockiness, rocky planets being the more Earth-like. They also accounted for a phenomenon called "eccentricity-albedo degeneracy," which comments on a sort of balancing act between the a planet's albedo—the energy reflected back to space from its surface—and the circularity of its orbit, which affects how much energy it receives from its .

The two counteract each other. The higher a planet's albedo, the more light and energy are reflected off to space, leaving less at the surface to warm the world and aid possible life. But the more noncircular or eccentric a planet's orbit, the more intense is the energy it gets when passing close to its star in its elliptic journey.

A life-friendly energy equilibrium for a planet near the inner edge of the habitable zone—in danger of being too hot for life—Barnes said, would be a higher albedo, to cool the world by reflecting some of that heat into space. Conversely, a planet near the cool outer edge of the habitable zone would perhaps need a higher level of orbital eccentricity to provide the energy needed for life.

Barnes, Meadows and Evans ranked in this way planets so far found by the Kepler Space Telescope, in its original mission as well as its "K2" follow-up mission. They found that the best candidates for habitability and life are those planets that get about 60 percent to 90 percent of the solar radiation that the Earth receives from the sun, which is in keeping with current thinking about a star's habitable zone.

The research is part of the ongoing work of the Virtual Planetary Laboratory to study faraway in the ongoing search for , and was funded by the NASA Astrobiology Institute.

"This innovative step allows us to move beyond the two-dimensional concept to generate a flexible framework for prioritization that can include multiple observable characteristics and factors that affect planetary habitability," said Meadows.

"The power of the habitability index will grow as we learn more about exoplanets from both observations and theory."

Explore further: Earth-like planets around small stars likely have protective magnetic fields, aiding chance for life

More information: "Comparative Habitability of Transiting Exoplanets." arXiv. arxiv.org/abs/1509.08922

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Torbjorn_Larsson_OM
3.7 / 5 (3) Oct 06, 2015
The index is on the right track because it promotes superhabitables (planets with long atmosphere lifetimes around stars with long main sequence lifetimes), and it pushes out Mars as too cold and Venus as too hot.

The best scoring planet on the index is the confirmed KOI 422b (IIRC), which is such a superhabitable. So now we have at least one such known planet, albeit being a KOI it is far away.

@Doug: The closest twin to us is Venus. It all depends on what you mean by "twin".

And the distance between us and an interesting planet doesn't matter much, as long as we can study it as a planet and - if it is inhabited - as an example of what it takes for life to emerge. Such planets will advance science tremendously, and we will all benefit.

If you must troll, instead of asking, and drag it down to bean counter level differences: benefits will range from knowing that there is other life in general, to advances in optics and data processing that benefits medicine especially.
Returners
1 / 5 (5) Oct 06, 2015
This is useless. You need to know things like age due to internal heating being relevant, composition, atmospheric pressure.

The habitability zone doesn't really matter if the planet is made of the wrong things: too much water restricting nutrient flow due to exotic ice formation (from pressure, not temperature,) too little water, too much sulfur or phosphorous, too little, etc, etc.

There's a lot more to life requirements than just temperature and liquid water.
Returners
1 / 5 (5) Oct 06, 2015
For like the billionth time, Venus is not a twin of the Earth.

That is bad science and a misnomer which has caused even highly educated people to draw totally wrong conclusions about the physics, history, and future of Venus and Earth, such as claims of run-away greenhouse effects raising the temperature on Earth by hundreds of degrees and so forth.

The claim that Venus was ever like Earth in he distant past is completely false, and when you examine it logically you will realize that Venus has always had a massive greenhouse effect, because it has always been as hot or hotter than it is now.
El_Nose
5 / 5 (1) Oct 06, 2015
@Torbjorn_Larsson_OM

I doubt Venus would get scaled as too hot-- as Venus reflects an awful lot of radiation. You would never know It was so hot if you did not know about the CO2 levels in the atmosphere that super heat the planet. Likewise Mars with almost no cloud cover - due to lack of dense atmosphere would get an incorrect score because it would seem to absorb all the radiation it can, but have no way to keep the heat.

That is why these scores are merely a guide and our own solar system give great counter examples.
Antonaccio
3.7 / 5 (6) Oct 06, 2015
@Returners

You must have a pretty good research paper that you can point me to about Venus if you're saying that it's completely false that the planet could have been more temperate in the past.
betterexists
1 / 5 (3) Oct 06, 2015
Life runs on tight budget; So, money should be well spent.
Rather they should Focus upon something different from Hyperloops for Internationally bound Airplanes!
I mean, first make sure Int'l Airports are near Sea shores and just Toss Planes with full force. Once that initial thrust is lost, the planes should be able to manoeuvre themselves on their own!
Returners
1 / 5 (5) Oct 06, 2015
@Returners

You must have a pretty good research paper that you can point me to about Venus if you're saying that it's completely false that the planet could have been more temperate in the past.


Nah. I just did my own calculations, knowing that Venus recieves about twice the Sunlight as the Earth, and has done so far as long as it has existed, and that molten planets are hotter than solid planets, which means that Venus was hotter in the past, and that magma certainly release CO2 and other gases much more quickly than does solid rock

Using a rough number, Venus' mean surface temperature would be around 600K even if it somehow otherwise had an Earth-like atmosphere. I admit I'd actually have to re-calculate the exact value again, since I haven't done it in a while, but this is close enough to make my point.

The myth of a past cooler Venus is just that: A fairy tale started by incompetent fools.
Nik_2213
5 / 5 (2) Oct 06, 2015
I notice they don't mention mega-moons of gas giants...
betterexists
1 / 5 (2) Oct 06, 2015
Life runs on tight budget; So, money should be well spent.
I mean, first make sure Int'l Airports are near Sea shores and just Toss Planes with full force. Once that initial thrust is lost, the planes should be able to manoeuvre themselves on their own!

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betterexists
1 / 5 (2) Oct 06, 2015
READ GOOGLE NEWS; $69 From U.S to Europe Soon I would love to see $ 0.69 fare!
Bob Osaka
5 / 5 (1) Oct 07, 2015
The article should read: "habitable to life as we know it." Certainly the oceans of Europa fall well outside of the Goldilocks zone.
Torbjorn_Larsson_OM
3.7 / 5 (3) Oct 12, 2015
Rereading the article, I found a problematic claim. It is AFAIK doubful that the Webb telescope can measure the atmospheric composition of a rocky planet. It will be able to study Jupiters atmospheres for sure.

@Bob: By convention the radiative habitable zone is shortened to HZ. There is an analogous tidal HZ Goldilocks zone around gas giants, where Europa fits.

While we are picking nits, "habitable to life as we know it" is a misnomer too. All the current HZ definitions imply is that it is describing the most likely HZs (if you include the water tidal HZ) and it doesn't go into details of the basis of hereditary et cetera.
Torbjorn_Larsson_OM
5 / 5 (2) Oct 12, 2015
@Returners: The article describes how the new habitable index is useful. And as I already mentioned, the case of twins depends on what you mean by "twin", which was my whole point in mentioning Venus. Venus is composition, mass and in some senses orbital twin to Earth. "In size and mass, it is similar to Earth, and is often described as Earth's "sister" or "twin".[19] " [ https://en.wikipe...ki/Venus ]

As for the rest, you are wrong and don't mention references that would back you up (re armchair 'science'). Instead you refer to 'calculations' that you don't show, that hasn't passed peer review, and is obviously based on erroneous assumptions of "incompetent fools". (Say, that the irradiation has been constant, or that "molten planets are hotter than solid planets" while we know Venus surface and atmosphere temperatures are greenhouse driven.)

[tbctd]
Torbjorn_Larsson_OM
5 / 5 (2) Oct 12, 2015
[ctd]

- "Studies have suggested that billions of years ago the Venusian atmosphere was much more like Earth's than it is now, and that there may have been substantial quantities of liquid water on the surface, but after a period of 600 million to several billion years,[56] a runaway greenhouse effect was caused by the evaporation of that original water, which generated a critical level of greenhouse gases in its atmosphere.[57]" [Ibid]

- "After another billion years all surface water will have disappeared[74] and the mean global temperature will reach 70 °C[78] (158 °F). Earth is expected to be effectively habitable for about another 500 Ma from that point,[73]" [ https://en.wikipe...d_future ]
Torbjorn_Larsson_OM
5 / 5 (2) Oct 12, 2015
@El Nose: I agree, it will be hard to study these worlds. That is why the indexes are a filter for initial screening while the effort of study is expensive.

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