News in Science

Old stars may still make new worlds

Building planets Stars considered too old to have new planets forming around them, could still be creating new worlds after all, according to a new study.

The research in the journal Nature used a new technique to measure the mass of the planet forming disk around a star called TW Hydrae, finding it contains enough material to make fifty Jupiters.

One of the papers authors, Dr Uma Gorti from NASA's Ames Research Centre and the SETI Institute, says the discovery is surprising because the star is about 10 million years old and shouldn't have enough material left in its disk to form large planets.

"This changes our understanding of how and when planets take shape around their stars," says Gorti.

As stars condense out of molecular gas and dust clouds, they produce flattened protoplanetary disks containing the raw materials which eventually clump together to form planets.

Astronomers assumed most of this material either falls onto the star, or is blown away by stellar winds in the first few million years of a star's life.

"TW Hydrae is about 176 light-years away, which is relatively close in astronomical terms," says Gorti.

"We've been studying it for some time because it has a gap in its protoplanetary disk which may have been caused by the formation of a planet."

"However it's difficult to accurately measure the amount of material left in the disk."

Heavy hydrogen

The new technique developed by Gorti and colleagues used the European Space Agency's Earth-orbiting Herschel space telescope to look for molecules of hydrogen deuteride.

Hydrogen molecules are the main gas component of planets, but they emit light at wavelengths too short to be easily detected.

Gas molecules containing deuterium, a heavier version of hydrogen, emit light at longer, far-infrared wavelengths which Herschel sees.

This enabled astronomers to precisely measure the levels of hydrogen deuteride in the disk.

The ratio of normal to heavy molecular hydrogen gas is well known from measurements in our local solar neighbourhood.

Gorti and colleagues were able to use the hydrogen deuteride readings to work out the total mass of the protoplanetary disk with ten times greater accuracy than ever before.

A lot to learn

Dr Simon O'Toole from the Australian Astronomical Observatory says the new technique means the amount of materials available for planet building in this system and others, has been underestimated.

"Our estimates of when and for how long planetary formation is likely to occur around a star is also out", says O'Toole.

"It tells us that we still have a lot to learn about planetary systems and their formation."

"Just when you think you're getting a handle on it, along comes a surprise like this."