Eric Agol, assistant professor of astronomy at the UW, wrote a paper published March 29 in The Astrophysical Journal Letters, pointing out that planets in orbit around white dwarfs would be easy to detect, if they exist, and that those planets might well be hospitable to life.
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| Eric Agol, assistant professor of astronomy at the University of Washington, suggests that white dwarf stars would be worth checking for habitable planets. Photo: UW. |
“The closest we typically see planets around stars is something like twice the Roche limit,” Agol explained. “It turns out that that distance for a white dwarf is .01 AU, and it’s right in the center of the white dwarf habitable zone.”
Stars like our Sun eventually become white dwarfs after going through their red giant phases and then shedding their outer atmospheres, leaving a hot, glowing core that has about 60 percent of the Sun’s mass but is about the size of Earth. Agol said if we look at white dwarfs any possibly habitable planets would be relatively easy to find.
“If an Earth were to pass in front of a white dwarf you would see a very deep dip due to the blockage of the light of the white dwarf by the planet, and that would be a signal that would be easy to detect from the ground,” he said. “You wouldn’t need to build an expensive space telescope.”
Agol describes his paper as “fairly speculative.”
“You need some mechanism of creating these planets after [a star’s] red-giant phase, or migrating them inwards from further out, and it’s not clear how often that should occur,” he said. He also allows that he may have been a bit on the generous side when penciling out the habitable zone around a white dwarf. The planets would likely be tidally locked to the stars, adding another complicating factor for life.
Agol figures that we might have to look at 20,000 white dwarfs to have a good chance to find a habitable planet. Since such planets would be so close to their stars, we wouldn’t have to look long, about 32 hours at most, to see a transit. Still, with that many observations, time adds up in a hurry.
Fortunately, Agol says there is a network of telescopes that may be up to the task. The Las Cumbres Observatory Global Telescope Network is a group about 20 one-meter robotic telescopes around the world, at sites in the northern and southern hemispheres. The key is that some of them are always in the dark, necessary to observe the same white dwarf for 32 hours. Even with this network, a white dwarf planet hunt would be an ambitious project.
“Even if you devoted those 20 telescopes to this sort of survey it would still take something like 15 calendar years,” Agol said. A wide-field scope could look at multiple stars at once, cutting down that time, and Agol says scopes such as the Large Synoptic Survey Telescope, in which the UW is involved, could help speed up observations of candidate stars.
Kepler grabs the fame, but Agol says that mission really isn’t much use for such a project. For one, it’s only looking at a small patch of sky. And it’s only able to see stars down to about 16th magnitude.
“White dwarfs out to 100 parsecs are going to be more down to like 19th magnitude,” Agol said of the collection of stars the survey would require. “[Kepler] doesn’t really have the sensitivity to look at stars faint enough to survey white dwarfs.”
