Looking like an image out of a video game, M 27, or NGC 6853 is a planetary nebula in the constellation Vulpecula, at a distance of about 1,360 light years. Its central star, a white dwarf, is larger than any other known white dwarf in the universe. Also known by the absurd name, the Dumbbell, it contains knots marked by a central region of dark and bright cusped knots and their associated dark tails. The knots vary in appearance from symmetric objects with tails to rather irregular tail-less objects. Similarly to the Helix Nebula and the Eskimo Nebula, the heads of the knots have bright cusps which are local.
White dwarfs, though born hot, typically have about 60 percent of the mass of the sun, but by volume they are only about the size of Earth. They eventually become cooler than the sun and emit just a fraction of its energy, so the habitable zones for their planets are significantly closer than Earth is to the sun.
"If a planet is close enough to the star, it could have a stable temperature long enough to have liquid water at the surface –- if it has water at all –- and that's a big factor for habitability," Agol said.
The nearest white dwarf to Earth is Sirius B at a distance of about 8.5 light years (a light year is about 6 trillion miles). It is believed to once have been five times more massive than the sun, but now it has about the same mass as the sun packed into the same volume as Earth.
Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-meter ground telescope, he said, one star could be surveyed in 32 hours of observation. If there is no telltale dimming of light from the star in that time, it means no planet orbiting closely enough to be habitable is passing in front of the star so that it is easily observable from Earth. Ideally, the work could be carried out by a network of telescopes that would make successive observations of a white dwarf as it progresses through the sky.
"This could take a huge amount of time, even with such a network," he said.
The same work could be accomplished by larger specialty telescopes, such as the Large Synoptic Survey Telescope that is planned for operations later this decade in Chile, of which the UW is a founding partner. If it turns out that the number of white dwarfs with potential Earthlike planets is very small –- say one in 1,000 –- that telescope still would be able to track them down efficiently.
Finding an Earthlike planet around a white dwarf could provide a meaningful place to look for life, Agol said. But it also would be a potential lifeboat for humanity if Earth, for some reason, becomes uninhabitable.
"Those are the reasons I find this project interesting," he said. "And there's also the question of, 'Just how special is Earth?'"
The Daily Galaxy via astrobio.net and ESO.org
Image Credit: http://www.sc.eso.org/~tszeifer/