Future evidence for extraterrestrial life might come from dying stars

Feb 25, 2013
This shows a habitable planet orbiting a white dwarf. Credit: David A. Aguilar (CfA)

(Phys.org)—Even dying stars could host planets with life—and if such life exists, we might be able to detect it within the next decade. This encouraging result comes from a new theoretical study of Earth-like planets orbiting white dwarf stars. Researchers found that we could detect oxygen in the atmosphere of a white dwarf's planet much more easily than for an Earth-like planet orbiting a Sun-like star.

"In the quest for extraterrestrial biological signatures, the first stars we study should be white dwarfs," said Avi Loeb, theorist at the Harvard-Smithsonian Center for Astrophysics (CfA) and director of the Institute for Theory and Computation.

When a star like the Sun dies, it puffs off its outer layers, leaving behind a hot core called a white dwarf. A typical white dwarf is about the size of Earth. It slowly cools and fades over time, but it can retain heat long enough to warm a nearby world for billions of years.

Since a white dwarf is much smaller and fainter than the Sun, a planet would have to be much closer in to be habitable with on its surface. A habitable planet would circle the white dwarf once every 10 hours at a distance of about a million miles.

Before a star becomes a white dwarf it swells into a red giant, engulfing and destroying any nearby planets. Therefore, a planet would have to arrive in the after the star evolved into a white dwarf. A planet could form from leftover dust and gas (making it a second-generation world), or migrate inward from a larger distance.

If planets exist in the habitable zones of white dwarfs, we would need to find them before we could study them. The abundance of heavy elements on the surface of white dwarfs suggests that a significant fraction of them have . Loeb and his colleague Dan Maoz (Tel Aviv University) estimate that a survey of the 500 closest could spot one or more habitable Earths.

The best method for finding such planets is a transit search - looking for a star that dims as an orbiting planet crosses in front of it. Since a white dwarf is about the same size as Earth, an Earth-sized planet would block a large fraction of its light and create an obvious signal.

More importantly, we can only study the atmospheres of transiting . When the white dwarf's light shines through the ring of air that surrounds the planet's silhouetted disk, the atmosphere absorbs some starlight. This leaves chemical fingerprints showing whether that air contains water vapor, or even signatures of life, such as oxygen.

Astronomers are particularly interested in finding oxygen because the oxygen in the Earth's atmosphere is continuously replenished, through photosynthesis, by plant life. Were all life to cease on Earth, our atmosphere would quickly become devoid of oxygen, which would dissolve in the oceans and oxidize the surface. Thus, the presence of large quantities of oxygen in the atmosphere of a distant planet would signal the likely presence of life there.

NASA's James Webb Space Telescope (JWST), scheduled for launch by the end of this decade, promises to sniff out the gases of these alien worlds. Loeb and Maoz created a synthetic spectrum, replicating what JWST would see if it examined a habitable planet orbiting a white dwarf. They found that both oxygen and water vapor would be detectable with only a few hours of total observation time.

"JWST offers the best hope of finding an inhabited planet in the near future," said Maoz.

Recent research by CfA astronomers Courtney Dressing and David Charbonneau showed that the closest habitable planet is likely to orbit a red dwarf star (a cool, low-mass star undergoing nuclear fusion). Since a red dwarf, although smaller and fainter than the Sun, is much larger and brighter than a white dwarf, its glare would overwhelm the faint signal from an orbiting planet's atmosphere. JWST would have to observe hundreds of hours of transits to have any hope of analyzing the atmosphere's composition.

"Although the closest might orbit a red dwarf star, the closest one we can easily prove to be life-bearing might orbit a white dwarf," said Loeb.

Explore further: Quest for extraterrestrial life not over, experts say

More information: Their paper has been accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online.

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User comments : 21

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Modernmystic
1.9 / 5 (15) Feb 25, 2013
Really? Look for signs of life around white dwarfs?? I thought they were stretching it looking around red dwarfs.

Do these people understand nothing about stellar evolution and habitability?
Sanescience
4.5 / 5 (2) Feb 25, 2013
(if) x (if) x (if) x (if) ... =? cant rule it out!

That said, it is probably pretty low on the priority list.
bachcole
4.1 / 5 (7) Feb 25, 2013
Remember also that white dwarfs last much longer than main sequence stars. There would also be less stellar variation on a white dwarf; wouldn't that make it easier for life to develop.
Modernmystic
2.7 / 5 (7) Feb 25, 2013
Remember also that white dwarfs last much longer than main sequence stars. There would also be less stellar variation on a white dwarf; wouldn't that make it easier for life to develop.


Any white dwarf is off the main sequence and has gone through a red giant phase. Any planet left close enough to the star to evolve life would have had its atmosphere stripped and sterilized during the giant phase....
antialias_physorg
4.8 / 5 (21) Feb 25, 2013
Oh my. An all-caps guy. Haven't had one of those particular crazies here in a while.
Q-Star
4.4 / 5 (13) Feb 25, 2013
* I have had the privilege of being recognized in the 24th edition of Marquis "Who's Who In The East" for my writings on religion and science,,,,,,,


Ya left out the part where ya paid a fee for the "privilege of being recognized".

antialias_physorg
3.9 / 5 (7) Feb 25, 2013
Any planet left close enough to the star to evolve life would have had its atmosphere stripped and sterilized during the giant phase....

I've been thinking about responding to that one for a few minutes - and in that time my answer has shifted to and fro. While generally I agree that an atmsophere would be stripped and the planet be left heavily irradiated I'm not sure whether an atmosphere could not reform (or whether an atmosphere is really such a necessary part of a life-bearing planet in the first place - given that we know of deep ocean life - and even deep rock life a mile under the Earth's crust)

Dwarf stars are incredibly long lived, and planetary orbits are also not as fixed over such timescales - so it could even be conceivable that some planet got shunted closer after the red giant phase.

But alltogether they'd not be at the top of my hit list of where to look first.
Modernmystic
3 / 5 (2) Feb 25, 2013
While generally I agree that an atmsophere would be stripped and the planet be left heavily irradiated I'm not sure whether an atmosphere could not reform (or whether an atmosphere is really such a necessary part of a life-bearing planet in the first place - given that we know of deep ocean life - and even deep rock life a mile under the Earth's crust)


Agreed, however this would be very speculative and I'd say not "mainstream astrophysics". I could be wrong on that one.

so it could even be conceivable that some planet got shunted closer after the red giant phase.


Agreed, on that part and I'd say it would be necessary as most terrestrial planets would be incinerated within the habitable zone of a white dwarf in its previous giant phase.

But alltogether they'd not be at the top of my hit list of where to look first.


Indded, but I do concede that there is an infinitesimal chance for the exact reasons you suggest. I wouldn't give a cent for looking there however.
kamil_muzyka
3 / 5 (6) Feb 25, 2013
yes there may be life on other planets, and yes - creationist loonies - it can originate from non-living matter. If god is so wonderful, maybe Evolution is the proof. my friend is a polish catholic and he doesn't exclude evolution, he states that in the genesis, there are lines when animal are created before man. God maybe a being, but he may be a personification of the forces of light and universe.

I'd really apreciate finding an extraterrestial life
form.
trekgeek1
4.8 / 5 (10) Feb 25, 2013

SCIENCE AND THE ORIGIN OF LIFE, NATURAL LIMITS OF EVOLUTION, HOW FORENSIC SCIENCE REFUTES ATHEISM, WAR AMONG EVOLUTIONISTS (2nd Edition), DOES GOD PARTICLE EXPLAIN UNIVERSE'S ORIGIN?


Atheism has nothing to do with cosmology. Similarly, people who do believe in a god can accept evolutionary theory which explains the passing of genetic traits between populations. Additionally, there is no war among evolutionary scientists. Scientists have never had a war amongst themselves over theories; that is something unique to religion. Finally, it's the Higgs Boson - playfully called the "God particle" because it was going to be called the "God Damned particle" because of its elusive nature. That was not appropriate for a scientific publication so it became the"God particle" among the scientifically illiterate.

TheGhostofOtto1923
2.8 / 5 (9) Feb 25, 2013
Any white dwarf is off the main sequence and has gone through a red giant phase. Any planet left close enough to the star to evolve life would have had its atmosphere stripped and sterilized during the giant phase....
I think that mm need only do a little research to determine the time is available after this phase for new planets to form as the article states, and for these planets to develop the conditions necessary for life.

And then he might appreciate how these scientists arrived at their conclusions.

"Therefore, a planet would have to arrive in the habitable zone after the star evolved into a white dwarf. A planet could form from leftover dust and gas (making it a second-generation world), or migrate inward from a larger distance."

-Jeez you guys arent even reading the articles any more-
gwrede
2.6 / 5 (5) Feb 25, 2013
gwrede
4.3 / 5 (6) Feb 25, 2013
It would be nice to have a delete button, or at least a check, that if you leave an empty comment, PhysOrg would understand that you got second thoughts.
FrankHerbertWhines
1.8 / 5 (5) Feb 25, 2013

best comment ever gwrede
aaron1960
3.8 / 5 (4) Feb 25, 2013
a white dwarf lasting billions of years gives plenty of time for new planets to condense and form afresh and evolve life of the kind as we know it.
Isaacsname
1 / 5 (1) Feb 26, 2013
I are an alien
Anorion
1 / 5 (4) Feb 26, 2013
white dwarf is much too faint and cold to support habitable plant
the plant would need to be very very close, and if its so close it would become tidally locked on the the white dwarf, cause even tough its much smaller and fainter and colder, it have retained most of its mass / gravity from its previous life , and any planet close enough to catch enough energy to be habitable, would be ripped apart by huge gravity or just tidally locked and always expose one side to light and other to night witch would make it unsuitable for life. and as some pointed out already , the red giant phase would have obliterate anything around it.life around white dwarf possible ? maybe .... but i wouldn't hold my breath for it. and even then probably would be only microbial extremophiles doweling in some particular "twilight" areas of the planet.
Isaacsname
not rated yet Feb 26, 2013
I are an alien


Laugh at my jokes or I will destroy you

I'm kidding

http://s839.photo...strappe/

Laugh at my jokes though
jacob_bush34
not rated yet Feb 26, 2013
It really caught my interest as I was browsing the r/science section of reddit. I try to stay updated on the search for extraterrestrial life, because I am so curious to see what else is out there. Without a doubt in my mind, I believe there is life outside our planet. The universe is simply too large to have only one planet with life on it. I found this discovery intersting because it could lead to something big. Any step in the search for extraterrestrial life interests me, but this could be a major breakthrough. It is a completely different approach to the search, but one has to take risks for them to pay off right? I am excited to see where this leads and thank you for writing such a cool article.
alfie_null
5 / 5 (1) Mar 03, 2013
No thoughts on the importance of molten cores? Regardless of the age of a sun, maybe planets have a limited period during which they can harbor life.
Graeme
not rated yet Mar 08, 2013
The close orbit, combined with any elliptical motion or rotation would cause tides that would heat the interior also. So lack of radioactive may not be an issue.

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