Star Not Too Old to Have Planets After All

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Star Not Too Old to Have Planets After All
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This artist's illustration shows a planetary disk (left) that weighs the equivalent of 50 Jupiter-mass …

Disks of matter thought to be too old to serve as planetary nurseries may still be capable of forming worlds, surprised researchers announced today (Jan. 30).

"This will lead to new ideas in planet formation theories," study lead author Edwin Bergin, an astrochemist at the University of Michigan at Ann Arbor, told SPACE.com.

Scientists analyzed TW Hydrae, a star 176 light-years from Earth in the constellation Hydra. TW Hydrae, which has about the same mass as the sun, is surrounded by a dense ring of gas and dust.  Such circumstellar rings are often protoplanetary disks, in which matter can clump to form larger rocks and eventually worlds. Since TW Hydrae is 2 1/2 times closer to Earth than the next nearest such star, astronomers studying planet formation have depended on TW Hydrae much as biologists do on lab mice, using it to help build computer models.

However, TW Hydrae itself was considered past its planet-forming years. Its circumstellar disk is estimated to between 3 million and 10 million years old, and most protoplanetary disks are thought to last only 2 million to 3 million years.

Through all the studies of TW Hydraw, a crucial detail of its disk remained uncertain: the total mass of the hydrogen molecules in it. This value is key to determining how many and what kinds of planets might form. Past estimates of the mass of TW Hydrae's disk ranged from as little as 160 times the mass of Earth to as much as 20,000 times, but the value could not be pinned down because regular hydrogen molecules do not emit detectable radiation. [9 Exoplanets That Could Host Alien Life]

To get around that problem, the researchers exploited the fact that not all hydrogen molecules are identical. A few are made up of one hydrogen atom and one deuterium atom instead of two regular hydrogen atoms. These "hydrogen deuteride" molecules have an extra neutron compared with regular hydrogen molecules, and they emit detectable amounts of far-infrared radiation based on how they rotate.

The ratio of deuterium to hydrogen appears constant in Earth's region of space, which means that measuring hydrogen deuteride would give investigators a good idea of how much regular molecular hydrogen is present.

The researchers used ESA's Herschel Space Telescope, which is sensitive to the required infrared wavelengths. They determined TW Hydrae's disk is at least 16,650 times the mass of the Earth. Considering the planets in the solar system may have arisen from a disk only as little as 3,300 times the mass of Earth, the matter in TW Hydrae's disk would be ample to form a planetary system.

"This points to the possibility that planet formation may not be a one-size-fits-all process," Bergin said. "It seems to point towards different systems finding disparate pathways to making planets."

"TW Hydrae is a good example of how a calculated scientific gamble can pay off," said study co-author Thomas Henning at the Max Planck Institute for Astronomy in Heidelberg, Germany. "At least one model predicted that we shouldn't have seen anything! Instead, the results were much better than we had dared to hope."

"If there's no chance your project can fail, you're probably not doing very interesting science," Henning added.

Signs of hydrogen deuteride remain difficult to detect around distant stars — this was only the second time it was seen outside the solar system, and the first time in a decade. For this kind of measurement to become a standard tool for understanding planetary formation, either a space-based telescope or an airborne observatory would be needed, Bergrin noted.

"There is some hope in the future that NASA's SOFIA observatory — an aircraft with a hole in it! — might be able to follow up on this result," Bergin said. "Over the longer term, Japan is exploring a space-based observatory that will be more sensitive than Herschel. That is in the planning stages and is called SPICA. If that flies, then this observation can become more routine."

"Going forward we have a new program using ALMA (the Atacama Large Millimeter/sub-millimeter Array in Chile) to provide even better temperature estimates and set more stringent constraints on the the disk gas mass," Bergin added.

The findings appear in tomorrow's (Jan. 31) issue of the journal Nature.

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