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Habitable exoplanets could exist at white dwarfs, or near dark matter

The views expressed are those of the author and are not necessarily those of Scientific American.


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White dwarf stars in the Milky WayAstronomers are probably just a few years from the first-ever finding of an Earth twin outside our solar system, that is, a planet roughly the size of Earth orbiting at a similarly temperate distance from a sunlike star. The idea is that finding Earths outside the solar system just might be a first step toward finding alien life. Earth has been exceptionally supportive of life-forms, so perhaps a similar planet would also be habitable.

NASA’s Kepler spacecraft is leading this charge, and has already sniffed out hints of planets that look remarkably Earth-like, at least from Kepler’s distant vantage point; it will take a few more years of observations to solidify these tentative findings. The spacecraft is keeping tabs on more than 150,000 stars to find out how often Earth-like worlds form around sunlike stars.

But sunlike stars may not be the only game in town, as far as habitability is concerned. In a study in the April 20 issue of The Astrophysical Journal Letters, astronomer Eric Agol of the University of Washington in Seattle raises the point that habitable exoplanets could also be found orbiting compact stellar remnants known as white dwarfs, which are plentiful in the universe. And if habitable worlds do exist around white dwarfs, Agol notes, it would be possible to detect them with modest-size telescopes on Earth, whereas finding Earth-like planets around sunlike stars requires a space-based telescope like Kepler, which cost NASA $600 million.

Kepler looks for tiny, periodic dips in a star’s brightness that might be caused by a planet passing across the face of the star and blocking some of its starlight, a sort of eclipse known as a transit. But those dips are extremely subtle—the transit of an Earth-like planet in front of a sunlike star dims the star by about 0.01 percent. With a much more compact white dwarf, an Earth-like world would blot out far more of its light. Agol calculates that an Earth twin in a potentially habitable orbit would block almost 50 percent of a white dwarf’s emitted light during a transit. A change of that magnitude would be detectable even with a one-meter telescope on the ground. For a true Earth analogue orbiting a sunlike star, on the other hand, the 0.01 percent dip can only be identified from space, outside the distorting, turbulent veil of Earth’s atmosphere.

The question is, can habitable planets even exist around a white dwarf? A planet would have to be exceedingly close to a white dwarf, which has exhausted its nuclear fuel and is quite dim, to receive enough heat to maintain liquid water on its surface. That wouldn’t be a problem—Kepler and other exoplanet searches have found many planets in very tight orbits—except for the fact that before a star becomes a white dwarf, it swells up as a red giant and swallows all the planets nearby. (That is the fate that may await Earth in a few billion years’ time when the sun becomes a red giant.) Perhaps planets could re-form after the ballooning red giant has retreated to become a much smaller white dwarf, or perhaps planets distant enough to escape immolation could migrate inward after the red giant phase ends. If such planets could take up orbits at the right distance from a white dwarf—about 0.5 percent to 2 percent the distance between Earth and the sun—they might have billions of years of temperate, potentially habitable conditions, according to Agol.

An even wilder idea is that dark matter, rather than starlight, could provide enough heat to make a planet livable. In a paper posted to the physics preprint Web site arXiv.org March 25, Dan Hooper and Jason Steffen of Fermi National Accelerator Laboratory in Batavia, Ill., note that large planets in dark matter–rich regions of the universe could gravitationally capture a substantial number of dark matter particles, which would mutually annihilate on contact. With a few optimistic assumptions of how dark matter behaves—remember that no one really knows what it is—the energy released in dark matter annihilations could provide a steady source of heat that might allow liquid water to persist on the surface of such a planet. "On these rare planets," Hooper and Steffen wrote, "it may be dark matter rather than light from a host star that makes it possible for life to emerge, evolve, and survive."

For the time being, astronomers have plenty of work to do following up Kepler’s leads for possible planets around ordinary stars. But if nothing else, the new studies help illustrate that we don’t really know the boundaries of what makes a planet habitable. With just one example of a livable world to draw from—our own—it makes sense to look for Earth-like planets first. But there might be an almost unimaginable combination of factors in this vast universe of ours that could render other kinds of worlds habitable as well.

Photo of ancient white dwarfs in the globular cluster M4: NASA and H. Richer (University of British Columbia)





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  1. 1. dbtinc 3:30 pm 04/1/2011

    hopefully, if discovered, they will be more intelligent than the current species here on earth. If they know what’s good for them, they’ll avoid contact at all costs.

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  2. 2. BoRon 3:31 pm 04/1/2011

    This sounds really far-fetched. A planet with life has its star go through the red giant phase and, then, magically migrates to a new orbit closer to the star, its life having somehow survived this process.

    Sounds like looking for your lost keys under a street light because it would be easier to see them there.

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  3. 3. Lowndes 4:15 pm 04/1/2011

    BoRon,

    Well said.

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  4. 4. DaMadScientist 4:45 pm 04/1/2011

    It didn’t say a planet with life magically migrates and somehow life survived. It could be a lifeless planet. The red giant could swell close to a dead planets orbit. Close enough to not boil away its oceans. Then when the star contracts it could pull that planets orbit in tighter. Now the once icy planet is in a close orbit with the white dwarf. Its liquid oceans could have 5 billion years to evolve life. Or maybe that planet started with life. The planets axis could be tilted in a way to have sections of the planet dark for long periods of time. The aliens could just stay in the shadows. It could even benefit them because the close hot sun roasting the planet is a major energy source. There are probably more than 100 billion * trillion planets. What seems unlikely is probably inevitable.

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  5. 5. BoRon 5:51 pm 04/1/2011

    "Then when the star contracts it could pull that planets orbit in tighter."
    The size of the star doesn’t affect the planet’s orbit. The mass does. As long as the star has the same mass, it could be a giant, main sequence, dwarf or black hole without changing the gravitational attraction/orbit. Of course, the star could grow so large that it engulfs the planet. That could slow the planet so it would fall in. So, if it slowed down just enough, it could fall in slower than the star contracted. But, then, the oceans would be gone.

    And, of course, there’s always the likelihood of the energy from dark matter scenario. That’s a good one.

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  6. 6. Daniel Rey M. 6:36 pm 04/1/2011

    "(…) at white dwarfs, or near dark matter"…or roving far and wide, neither attached to a star nor surrounded by merely hypothetic "dark matter", as suggested by Harlow Shapley in an essay nearly half a century ago.

    It’s titled "Life Among the Dwarfs" (Beyond the Observatory, Scribners, N.Y., 1967) and in it he describes his "Brobdingnagian planets" (cp. Gulliver’s Travels), fifty times as massive as Jupiter, where "the heat of gravitational compression would provide a warm surface". The energy that life requires would show up at the surface mainly as radio waves, so that creatures there would have radiowave vision.

    There are at least two possible objections. First, planetary science has progressed and now massive objects that hefty are classified as red dwarfs, but maybe warm surfaces provided by gravitational forces are feasible lower down the scale, and second, it seems like radiowave eyes that could furnish clear images would have to be as big as a bus, due to the wavelength of radio waves, which explains the huge size of radiotelescopes (whereas visible-light waves are so short that even the eyes of a baby shrew or mouse can accommodate them).

    However, "transformation optics", a yet to be developed technology that would be able to channel photons by manipulating the electric and magnetic fields of light, hints at a way around the latter problem. No longer would enormous eyes would be necessary. Nature might’ve found a way to funnel radio waves into tiny ones.

    In that case, as we to continue to search for life light-years away, ET life could sneak up on us and then vanish forever unless we’re ready for a round-trip mission to the visitor at a moment’s notice. This wouldn’t be easy since it would have to take into account the overwhelming gravitation and the presence of hostile microorganisms. Last January I proposed one such mission to a derelict planet to the space agencies but got no replies, yet the discussion having to do with dark matter here sounds even crazier than this.

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  7. 7. Daniel Rey M. 6:41 pm 04/1/2011

    It’s odd that leaders (sets of three periods) are turned into ampersands when comments are sent.

    Shapley died recently. He’s the astronomer who discovered we’re off-center in the galaxy, decades ago.

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  8. 8. jtdwyer 7:37 pm 04/1/2011

    I agree that Newton’s gravitational equation does not consider the density of mass (I’m not so sure about GR’s), but I think any such effect would likely only be relevant for proximal masses. This may explain the inability of Newton’s equations to predict Mercury’s daisy petal progression around the Sun.

    In any case, as I understand, a Star transitioning from the red giant to white dwarf phase loses a great deal of mass, so I agree that it exceedingly unlikely that "when the star contracts it could pull that planets orbit in tighter."

    Don’t proposals such as this require compelling evidence?

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  9. 9. byronraum 11:10 pm 04/1/2011

    The red giant loses a lot of matter in the process of becoming a white dwarf. This could form into planets – so planets could form after the formation of the white dwarf. There’s still enough time for life to evolve on the newly formed planet. However, please keep in mind that it will soon end up tidally locked.

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  10. 10. byronraum 11:34 pm 04/1/2011

    It also occurs to me that there is no solar wind; the white dwarf supplies only radiation. In order to have evolution, you must have mutation, and in order to have mutation, you must have variability. With a tidally locked planet with a constant source of unchanging heat, it’s going to be harder to get change.

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  11. 11. jtdwyer 12:18 pm 04/2/2011

    Good points, but how much is planetary accretion connected to stellar accretion? Since 99.86% of Solar system mass ended up within the Sun, could planets have accreted by themselves? Don’t observed planetary nebulae tend to remain in gaseous nebula form rather than producing another accretion disc?

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  12. 12. byronraum 10:03 pm 04/2/2011

    I am not sure about planetary nebulae. You are quite right in that they are, of course, in gaseous form, but it occurs to me that I cannot think of any good reason as to why a part of the the gas in a planetary nebula cannot accrete to form a planet. According to the wikipedia entry, "In recent years, Hubble Space Telescope images have revealed many planetary nebulae to have extremely complex and varied morphologies." Of course, all the matter is moving outwards from the star but the complexity would both indicate a possibility of greater density and that not all material is moving in the same direction. At some point, the greater density could conceivably start accreting into a planet-like body. All possibilities, unfortunately, no way of knowing for sure until we can confirm.

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  13. 13. Johnay 10:48 pm 04/2/2011

    If the outer fringes of a star expanding to become a red giant expand into the orbit of a planet, IIRC the portion of the star beyond the orbital radius would have effectively zero gravitational pull, thus reducing the pull on the planet. And then there’s the friction of the planet passing through the outer fringes of the star. What would be the combined effect? Would the planet end up thrown outwards or fall inward? If the former, what kind of orbit would it go into? How would that affect the orbits of other planets in the system? Could it then interact with one or more other planets, perhaps setting one of them on a course that settles it in a closer orbit around the time the star shrinks?

    Or planets could collide, with new planets eventually coalescing from their debris.

    Or there’s the possibility of a planet being captured by the system after the star cools.

    Lots of things could happen that could put a potentially life-sustaining planet in a close orbit around a white dwarf, and there are a lot of white dwarfs.

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  14. 14. jtdwyer 8:47 am 04/3/2011

    Still, of all the things that could happen in the production of a white dwarf star, there seems to be little reason to think that a probable outcome is the production or migration of a large planet to an orbit near the dwarf star.

    The principal benefit of searching for such a planet is that if one does exist, it should be easily detectable. I see little reason to think that life should likely be found on such a planetary system…

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  15. 15. jtdwyer 5:41 pm 04/3/2011

    I won’t even comment on the dark matter suggestion…

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  16. 16. jtdwyer 5:41 pm 04/3/2011

    I won’t even comment on the dark matter suggestion…

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  17. 17. BoRon 5:57 pm 04/3/2011

    "Don’t proposals such as this require compelling evidence?"

    I really hate deciding because I’ll have to create a new universe in which I answer the other way. And, it’s my day of rest.

    Link to this

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