Dying Star Builds Planets

A color image of the Mira system constructed from an archival Hubble Space Telescope image. Mira B glows blue because of the violent shock front formed as the inner part of the disk crashes into the central star. The dust outflow from Mira A has a green color because it is nearly-transparent emission from sillicate dust, and the more red color from near Mira B is due to the opaque edge of the disk being heated by Mira A. CREDIT: Caltech.

SEATTLE - Dust shed by a dying star is being siphoned by its healthy companion to build new planets, a new study suggests.

Located 350 light-years away in the constellation Cetus, Mira A is a massive, bloated red giant star that was once very similar to our Sun. It is now in its final stages and on its way to becoming a stellar corpse known as a white dwarf, a process that will take about a million years.

As part of this transformation, Mira A is shedding its outer layers of dust, at a rate of about one Earth-mass every seven years [image]. Normally, this material would diffuse into outer space.

Captured instead

But using the Keck I telescope in Hawaii and the Gemini South telescope in Chile, researchers have discovered that about 1 percent of this dust is being gravitationally captured by Mira A's companion, Mira B. The two stars are separated by a distance of about 90 AU. One AU, or astronomical unit, is equal to the distance between the Earth and the Sun [image].

Mira A's dust is settling into a ring around Mira B, a less massive version of our own Sun, and could eventually serve as the building material for new planets. The shed star dust consists mostly of silicate, the same material that makes up the Earth's mantle.

"This is a new type of planetary disk that is born when a companion star dies," said study team member Michael Ireland of Caltech. "At the moment, the disk is probably less than a Jupiter-mass in size. But over the lifetime of Mira A, it's going to accrete three to five times the mass of Jupiter."

The researchers spotted Mira B's disk because the part of the disk facing Mira A glowed brightly in the infrared, heated by radiation from the red giant. Ireland presented his team's findings here this week at the 209^th meeting of the American Astronomical Society.

Death meant death

Astronomers used to think that the death of a star meant the death of its planets, but in the case of Mira A, an "aging star is laying the foundation for a new generation of planets," Ireland said.

"This discovery opens up a new way to search for young planets, by searching in double star systems that contain white dwarfs," Ireland said. "The expected abundance of these systems means that we can find planets that we know are young around stars like our Sun."

If planets were to form around Mira B, and life developed on those planets, radiation from Mira A is not expected to pose a threat once enough time passes.

"It would pose a problem for life forms during the planetary formation process, while the white dwarf is still very young. But the white dwarf cools very quickly," Ireland said.

When Mira A contracts into a white dwarf, it will be about the size of the Earth, separated from Mira B by a distance equal to that between Earth and Pluto, and it will shine less brightly than a full Moon in our night skies.

"It won't affect life," Ireland said.

Editor's Note: All week, SPACE.com is providing complete coverage of the 209th meeting of the American Astronomical Society.

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