TW Hydrae Protoplanetary Disk Weighed With Unprecedented Accuracy
January 30, 2013

Astronomers’ Favorite Planetary Nursery Gets Weighed With Unprecedented Accuracy

Lee Rannals for - Your Universe Online

Astronomers recently reported in the journal Nature that they have determined the mass of the planetary nursery surrounding the star TW Hydrae with unprecedented accuracy.

TW Hydrae lies roughly 176 light-years away from Earth and is one of the most frequently observed protoplanetary disks in space. The young star is surrounded with a disk of dense gas and dust, in which larger objects begin to turn into planets. Our Solar System entered a similar phase about 4 billion years ago.

New measurements taken by astronomers using the European Space Agency's (ESA) Herschel Space Telescope shows a much larger mass for TW Hydrae's disk than what had previously been measured. These new observations could mean that the system is forming planets similar to those found in our own Solar System.

“The tiny solids in the disk that we measure are the seeds of terrestrial worlds — the stuff the Earth is made of ground into tiny little pieces, smaller than the width of a human hair,” Ilsedore Cleeves, a University of Michigan doctoral student who coauthored the paper, explained in a statement.

Edwin Bergin, professor of astronomy at University of Michigan and lead author of the paper, explained this research gives astronomers a better grip on the question "Why are we here?"

“If you want to understand the origin of planets — that is, terrestrial worlds like Earth with abundant water and life and gas-rich worlds such as Jupiter — you have to understand how planets are born and what outcomes are possible under any circumstances," Bergin explained. “And the mass of the protoplanetary disk is a fundamental quantity you have to have in order to understand planetary birth.”

Because of TW Hydrae's proximity to Earth, astronomers have an excellent view at two-and-a-half times closer than the next nearest similar objects. Despite it being so frequently observed, the total mass of the molecular hydrogen gas contained within the disk is a fundamental parameter that had remained unclear.

Astronomers have had difficulties gauging the mass of protoplanetary disks. Previous observations led to other model assumptions, but these results were littered with errors. However, the latest measurements bring a little more understanding to the equation.

“We are sensitive to the dust when it´s tiny. When it´s forming planets, it has to grow from sub-micron to pebbles to boulders," Cleeves said. "When the grains become boulders, we can´t detect them anymore. You could hide tons of mass that way."

The astronomers found that not all hydrogen molecules are created the same, and some contain a deuterium atom — also known as ℠heavy hydrogen´ on account of the fact that it carries both a proton and neutron in its nucleus. This change means these "hydrogen deuteride" molecules consist of one deuterium, and one ordinary hydrogen atom, changing the mass.

"We realized that Herschel was our only chance to observe hydrogen deuteride in this disk — way too good an opportunity to pass up," said Thomas Henning, director of the Department of Planet and Star Formation at the Max Planck Institute and coresearcher on the project. "But we also realized we would be taking a risk. At least one model predicted that we shouldn't have seen anything! Instead, the results were much better than we had dared to hope."

The researchers said this is just the second time hydrogen deuteride has been detected outside of our Solar System. The first was an observation in the Orion nebula taken by the ISO satellite.

New observations set a lower limit for the disk mass at 52 Jupiter masses and dramatically decreased the degree of uncertainly. The Max Planck Institute for Astronomy pointed out that the latest research shows there is more matter in the disk for forming planetary system than in our own.

"Our measurement shows that this disk is capable of making a solar system in some ways quite different from our own," Bergin said.

The latest mass measurements exploit the fact that hydrogen deuteride emits radiation associated with rotational degrees of freedom, which is a million times stronger than ordinary molecular hydrogen.

“Our findings tell us that different stars have different routes to making planets and that planet formation is not a one-size-fits-all process. We already know that our solar system is not alone and that there are systems that have more massive planets such as Jupiter." Bergin said.