Team catalogs most likely 'second-Earth' candidates

August 3, 2016, San Francisco State University
Kepler-186f, seen here in an artist's rendering and discovered in 2014 by a team of astronomers including SF State's Stephen Kane, is one of more than 200 "exoplanets" that researchers say lie within the "habitable zone" of their stars and could potentially have life. Credit: Danielle Futselaar

Looking for another Earth? An international team of researchers has pinpointed which of the more than 4,000 exoplanets discovered by NASA's Kepler mission are most likely to be similar to our rocky home.

The research, detailed in an article to be published in the Astrophysical Journal, outlines 216 Kepler planets located within the "habitable zone"—the area around a star in which a planet's surface could hold liquid water. Of those, they list 20 that are the best candidates to be habitable rocky planets like Earth.

"This is the complete catalog of all of the Kepler discoveries that are in the habitable zone of their host stars," said Stephen Kane, an associate professor of physics and astronomy at San Francisco State University and lead author of the study. "That means we can focus in on the planets in this paper and perform follow-up studies to learn more about them, including if they are indeed habitable."

The research also confirms that the distribution of Kepler planets within the habitable zone is the same as the distribution of those outside of it—additional evidence that the universe is teeming with planets and moons where life could potentially exist.

The boundaries of the habitable zone are critical. If a planet is too close to its star, it will experience a runaway greenhouse gas effect, like Venus. But if it's too far, any water will freeze, as is seen on Mars. Kane and his colleagues sorted the planets by whether they were in a conservative or a more optimistic interpretation of the habitable zone. Then they further sorted them by planet size: smaller, versus larger gas giants.

This figure shows the habitable zone for stars of different temperatures, as well as the location of terrestrial size planetary candidates and confirmed Kepler planets described in new research from SF State astronomer Stephen Kane. Some of the Solar System terrestrial planets are also shown for comparison. Credit: Chester Harman

The four categories are aimed at helping astronomers focus their research. Those looking for moons that could potentially hold life can study exoplanets in the gas giant categories, for example.

The 20 planets in the most restrictive category—rocky surface and a conservative habitable zone—are the most likely to be similar to Earth. Kane has already begun gathering additional data on these planets, as well as those in the other categories.

Studying and cataloging the more than 4,000 exoplanets took more than three years and involved researchers at NASA, Arizona State University, Caltech, University of Hawaii-Manoa, the University of Bordeaux, Cornell University and the Harvard-Smithsonian Center for Astrophysics. The team included Natalie Hinkel, a former SF State post-doctoral scholar now with Arizona State, and Michelle Hill, an undergraduate Australian student studying abroad at SF State.

"It's exciting to see the sheer amount of planets that are out there, which makes you think that there is zero chance of there not being another place where life could be found," said Hill, who sought out Kane shortly after arriving on campus and soon got involved with the research.

Kane is one of the world's leading "planet-hunters," having discovered several hundred exoplanets, including Kepler-186f, a rocky planet included in the new catalog. He is contributing his scientific knowledge to two upcoming satellite missions - NASA's Transiting Exoplanet Survey Satellite (TESS) and the European Space Agency's Characterizing ExOPLanet Satellite (CHEOPS)—that will benefit from the information contained in his latest research.

"There are a lot of planetary candidates out there, and there is a limited amount of telescope time in which we can study them," Kane said. "This study is a really big milestone toward answering the key questions of how common is life in the universe and how common are like the Earth."

"A catalog of Kepler exoplanet candidates," by Stephen R. Kane, Michelle L. Hill, James F. Kasting, Ravi Kumar Kopparapu, Elisa V. Quintana, Thomas Barclay, Natalie M. Batalha, William J. Borucki, David R. Ciardi, Nader Haghighipour, Natalie R. Hinkel, Lisa Kaltenegger, Franck Selsis and Guillermo Torres, can be read online at http://arxiv.org/abs/1608.00620 and will be published in an upcoming print edition of the Astrophysical Journal.

Explore further: Video: The diversity of habitable zones and the planets

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19 comments

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jonesdave
4.6 / 5 (10) Aug 03, 2016
From what I recall of the Kepler mission, it used the transit method to detect the planets. Given that these particular planets will have short orbital periods, then it is not beyond the realms of possibility that current or near future instruments could get decent spectroscopic data from the planetary atmospheres, as they transit on a regular basis.

Direct download link for the preprint: http://arxiv.org/...20v1.pdf
Captain Stumpy
4.6 / 5 (10) Aug 03, 2016
it is not beyond the realms of possibility that current or near future instruments could get decent spectroscopic data from the planetary atmospheres
@jonesdave
you know, i asked this about a specific planet a few months back... talked to a Ph.D. candidate in the Department of Astronomy & Astrophysics at the Penn State who's main research interest is extra-solar planets

i just asked permission to share what she said (and i also asked her to expound a bit further)

I hope to get a reply in the near future: she's always been great about communicating

If you want, perhaps you can pose the same questions to one of the authors?
or let me know and i will e-mail them
(kinda in a hurry this morning- Honey-do list)

i will post her reply as soon as i get it
jonesdave
4.5 / 5 (8) Aug 03, 2016
@CS,
Would certainly be interested to hear what is currently possible. My understanding from the literature is that it is a little way away for close in planets. Not too far, though. Found this article, which sums things up pretty well: http://www.nature...-1.19388
Captain Stumpy
4.5 / 5 (8) Aug 03, 2016
@CS,
Would certainly be interested to hear what is currently possible. My understanding from the literature is that it is a little way away for close in planets. Not too far, though. Found this article, which sums things up pretty well: http://www.nature...-1.19388
@jonesdave
cool... that is one article we discussed in the live-cast which lead me to ask about the atmosphere of the planet under discussion

let me see if i can't dig up the live-cast as she actually answers part of the question in it... it's just about a specific planet, mind you

it does, however, indicate some of the problems we will have for all the exoplanets we will be looking at

give me a little time to search
headed out again

here is a related topic though: CME measurement on exoplanets magnetic fields
https://youtu.be/...g?t=2112
Captain Stumpy
4.6 / 5 (10) Aug 03, 2016
WOW! already got an answer!
it's long, so bear with me
from Kim Cartier (the astrolady herself! http://sites.psu....trolady/ )
Thanks for emailing me, and I'm glad to help out! This is going to be a long answer, because planetary atmospheres are very complicated to measure.

Your friend is partially correct, in that there does exist the potential for spectroscopic measurements of the atmospheres of some of HZ exoplanets catalogued by that paper. Many system parameters contribute to the overall probability of measuring an atmosphere. For the case of all HZ planets reported there, you would measure the atmosphere using transmission spectroscopy, which observes the starlight as it passes through the planetary atmosphere. The other technique for measuring the atmosphere, emission spectroscopy, only works if your planet it hot enough for us to detect the thermal emission from the planet compared to the star.
2Bcont'd
Captain Stumpy
4.6 / 5 (10) Aug 03, 2016
Cont'd
A planet hot enough for that would definitely not be in the HZ, so we wouldn't use it for these HZ planets.

For transmission spectroscopy, detection of the atmosphere is dependent on how extended the atmosphere is above the planetary surface, and how dense that atmosphere is. A more extended atmosphere will leave a larger signature (exactly like a larger planet will leave a larger transit signature), and a dense atmosphere will leave more of a signature in the starlight.

Given what we know about these planets and the stars they orbit, here's what we can say:

-planet temperature: we've already negated the effects of this parameter by pre-selecting only HZ planets. Generally, a hotter atmosphere will be more extended (puffier), but with all planets in approximately the same temperature range (HZ) this makes little difference to this sample of planets.
2Bcont'd
Captain Stumpy
4.6 / 5 (10) Aug 03, 2016
cont'd
- planet size or mass: a larger planet is more massive, which means that it will likely have enough mass to attract an atmosphere while forming. But the more massive a planet, the tighter it holds to its atmosphere, so the atmosphere is less extended and harder to detect. But also, that would make the atmosphere more dense, which make the signature stronger. There's a delicate balance there, and a narrow range of planet sizes/masses that work with current technology. Overall, if the planet is a gaseous giant planet, there's enough atmosphere that this makes no difference, and we would be able to detect the atmosphere. The range of usable planet sizes gets narrower with rocky planets.

- orbital period: orbital period has a different impact than expected when we've already pre-selected HZ planets. Good: shorter orbital period lets us repeat measurements more easily and bump up the signal-to-noise.
2Bcont'd
Captain Stumpy
4.6 / 5 (10) Aug 03, 2016
cont'd
Also, short orbital period in the HZ by definition means orbiting a low-mass, low-temperature star, which would make the transit and atmospheric signature larger because the star is smaller. Bad: orbiting close to a low-mass, low-temperature stars makes atmospheres more susceptible to stellar flares and harmful UV radiation that might destroy detectable molecules in the atmosphere. Also, low-mass stars are sometimes cool enough to have their own molecules in their atmosphere, some of which are very hard for us to predict and identify in their spectra. If we're looking for precise atmospheric measurements of the planet's atmosphere, we need a stellar spectrum which we know very well, which is difficult for low-mass stars. Again, orbital period can both help and hinder atmospheric measurements.
2Bcont'd
Captain Stumpy
4.7 / 5 (12) Aug 03, 2016
last post
I hope this helps! Again, sorry about the length, but atmospheres are such a new field and really at the limits of our current technology that the combination of parameters that we can detect is very limited.
now that is what i call great feedback!

this is just one more reason I love science: those who are in the field doing research are usually willing to give feedback and answers to those who are interested in it what they are doing

A big thank you to (Soon-to-be Dr.) Kim Cartier and her awesome research in this very field (exoplanets)
http://sites.psu....trolady/

and a thanks to Fraser Cain for the introduction and letting us all enjoy her knowledge via live-casts on youtube
for those interested, check it out after the summer hiatus
https://www.youtu...iWolyvXp
tblakely1357
2.9 / 5 (7) Aug 03, 2016
From what I've read and seen, a habitable planet for our type of life needs:
1) The right chemistry
2) Be in the 'goldilocks' zone from the star
3) Have a stable star
4) Have a strong magnetic field
5) Have it's solar system in the 'goldilocks' zone of the galaxy
6) Have a large moon or orbit a giant planet for a stable climate (needed for more complex lifeforms)
7) Be not too close to giant stars that will supernova

And I'm sure there are other criteria I've left out.
Mark Thomas
5 / 5 (10) Aug 03, 2016
CS, my personal thanks to you for your successful efforts to add something to the science being described. So many of these threads degenerate into flame wars (or whatever you call them), that I find it discouraging to even mention a possible alternative or nuance when folks are calling each other morons or the like.

Regarding the content of the message and looking at all the confounding factors, I have to wonder if Kim Cartier believes this will eventually drive us towards large direct imaging space telescopes. For example, the High Definition Space Telescope (HDST) is getting a lot of mentions recently, but it will be anything but cheap.
jonesdave
4.4 / 5 (7) Aug 03, 2016
@CS,
Thanks for that. Very interesting. Kind of confirms what I thought; it won't be easy, but not impossible. And there are people who are working on it. Good news.

A nice little break from the mad Io thread that has been occupying my time!
torbjorn_b_g_larsson
4.3 / 5 (6) Aug 03, 2016
It is very interesting to see the habitable planets even if Kepler candidates are too far away to be easy (or at all) to follow up. Kepler was a medium range survey so planets are mostly 1 - 3 kyrs away. TESS will go up and survey all the closer planets that can be seen in transit, to locate the ones where observatories can detect life soon.

The paper is a bit tedious since they list the old type of candidates. They then complement with the false positive probability (FPP) that now sorts all Kepler pipeline planets. So some of the 20 have too large (> 5 %) FPP to be planets.

Good news, 3 planets are assuredly small enough to not be gas planets, and likely some more isn't. That adds to the 2 earlier, closer in assured habitables that was found by ground based observatories (HARP, IIRC).

Thanks for an informative thread, people!
torbjorn_b_g_larsson
4.2 / 5 (5) Aug 03, 2016
From what I've read and seen, a habitable planet for our type of life needs:
1) The right chemistry
2) Be in the 'goldilocks' zone from the star
3) ...


The current habitability filtering is based on 1) by looking for small, non-gas giant planets and 2) looking for planets within the surface habitable zone (sHZ). Arguably "our type of life" could be found in ice moons, but we have only our own system to assess that.

As we move forward, the filter will become better, but the planet list is a start for doing those very observations. The other factors you list are arguable, so while I wouldn't say it is too early to speculate, it isn't very helpful without quantification. Some are on that, and those are interesting preparations when looking for biosigns.

Nitpick: Your criteria 6) and 7) are (obviously, for 7)) wrong.
torbjorn_b_g_larsson
4.2 / 5 (5) Aug 03, 2016
Errata: "(HARP, IIRC)" - HARPS.
tblakely1357
2 / 5 (4) Aug 03, 2016
If a planet doesn't have a big moon or orbit a larger planet, it's axis will wobble greatly and cause very rapid and dramatic climate change. While that probably won't affect single cell life that much it will pretty much doom complex lifeforms.

If a planet is too close to a supernova, the radiation from the supernova could sterilize the planet. This is especially true if the planet is looking down the 'bore' of a GRB.

Not really sure how any of the above is wrong or even controversial.
Captain Stumpy
4.2 / 5 (5) Aug 03, 2016
So many of these threads degenerate into flame wars (or whatever you call them)
@Mark Thomas
Yep, that's what it's called
everyone will eventually degrade at some point, especially when the troll/religious fanatic/pseudoscience advocate repeats the exact same lie over and over ad nauseum
(sorry if i do, but i am especially intolerant of blatant pseudoscience/religious stupidity)
Regarding the content of the message and looking at all the confounding factors, I have to wonder if Kim Cartier believes this will eventually drive us towards large direct imaging space telescopes
well, you can ask her (her web-site link is above) but it seems to me that she does

I happen to be very optimistic about it myself - but wonder about resolution

.

Very interesting. Kind of confirms what I thought
@jonesdave
yeah... can't find the other vid i wanted to link, but considering i got feedback from Kim Cartier, i don't feel too bad
Tony Lance
5 / 5 (1) Aug 04, 2016
566 Exoplanet solar systems with orbital resonances and K2 todate.
http://www.bigber...ets.html
torbjorn_b_g_larsson
3 / 5 (2) Aug 05, 2016
If a planet doesn't have a big moon or orbit a larger planet, it's axis will wobble greatly and cause very rapid and dramatic climate change.


No, it won't. A few years ago, the scientists discovered an error in the algorithms that were used to predict that. So a body like Mars for example may have 0.5 Gyrs between large tilts that change climate, which is enough time to evolve land animals, say.

the radiation from the supernova could sterilize the planet.


Future possibilities has nothing to do with studying current habitability..

Obviously.

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