To find life, search near dying stars

U. WASHINGTON (US) —The best place to look for planets that can support life may be around dim, dying stars called white dwarfs.

Believed to be in the final stage of life, white dwarfs typically have about 60 percent of the mass of the sun, but by volume they are only about the size of Earth. Though born hot, 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,” says Eric Agol, associate professor of astronomy at the University of Washington and author of a new study published in The Astrophysical Journal Letters.

Agol is proposing a survey of the 20,000 white dwarfs closest to Earth. Using a 1-meter ground telescope, he says, one star could be surveyed in 32 hours of observation. 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.

White dwarfs evolve from stars like the sun. When such a star’s core can no longer produce nuclear reactions that convert hydrogen to helium, it starts burning hydrogen outside the core.

Finally the star sheds its outer atmosphere, leaving the glowing, gradually cooling, core as a white dwarf, with a surface temperature around 5,000 degrees Celsius (about 9,000 degrees Fahrenheit). At that point, the star produces heat and light in the same way as a dying fireplace ember, though the star’s ember could last for 3 billion years.

Once the star sheds its outer atmosphere, more distant planets that were beyond the reach of that atmosphere could begin to migrate closer to the white dwarf, Agol says. New planets also possibly could form from a ring of debris left behind by the star’s transformation. In either case, a planet would have to move very close to the white dwarf to be habitable, perhaps 500,000 to 2 million miles from the star. That’s substantially closer than Mercury is to the sun.

“From the planet, the star would appear slightly larger than our sun, because it is so close, and slightly more orange, but it would look very, very similar to our sun,” Agol says.

The planet also would be tidally locked, so the same side would always face the star and the opposite side would always be in darkness. The likely areas for habitation, he said, might be toward the edges of the light zone, nearer the dark side of the planet.

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.

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