The young star TW Hydrae has long been a favorite astronomical target as it is the nearest star to the solar system to play host to a protoplanetary disk — the gas and dust collected around a young star that goes on to form planets. It’s akin to an astronomer’s Petri dish — all the ingredients are there for the formation of planets and it’s ‘only’ 176 light-years away, a prime location for our telescopes to study.
However, TW Hydrae’s protoplanetary disk has been too small to image directly and its size can only be inferred through spectroscopic analysis (i.e. astronomers analyze the light from the system, deducing what material it contains). Now, with the help of Europe’s Herschel Space Observatory and the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, a team of astronomers have made a lucky discovery. Not only have they gained a more precise estimate of the mass of the protoplanetary disk, they’ve also found that it may produce a system of worlds of comparable mass to the solar system.
Yes, TW Hydrae may look like the solar system did over 4 billion years ago.
Previous mass estimates came from model assumptions of the star’s protoplanetary material — creating huge margins for error — but Herschel’s sensitivity to a certain wavelength of radiation from TW Hydrae was a game changer.
Contained within the protoplanetary disk are hydrogen molecules, but some contain deuterium (a hydrogen atom with an additional neutron in its nucleus). These “hydrogen deuteride” molecules emit a very specific radiation and because Herschel has such a fine-tuned resolution to this specific wavelength, astronomers were able to use this radiation as a guide. ALMA provided key information about the temperature of the protoplanetary disk by detecting the temperature of carbon monoxide. By combining these data, a precise ratio of hydrogen deuteride to hydrogen could be made — a mass estimate could therefore be attained.
It turns out that the baby TW Hydrae system is more massive than other estimates predicted. Rather than ranging anything from 0.5 to 63 Jupiter masses, the material contained within the protoplanetary disk has a lower limit of 52 Jupiter masses — this is more material than what formed our solar system during its formative years. Perhaps TW Hydrae will eventually host a similar system of planets as our sun did.
“This project started in casual conversation … We realized that Herschel was our only chance to observe hydrogen deuteride in this disk — way too good an opportunity to pass up. 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,” said Thomas Henning, of the Max Planck Institute for Astronomy in Heidelberg.
“If there’s no chance your project can fail, you’re probably not doing very interesting science. TW Hydrae is a good example of how a calculated scientific gamble can pay off.”
Interestingly, although TW Hydrae is a “young” star in cosmic terms, it is also a star that would normally be considered too mature to host a protoplanetary disk.
“We did not expect to find so much gas around this 10-million-year-old star,” said lead researcher Edwin Bergin of the University of Michigan.
“This star has significantly more mass than required to make our own solar system and could make a much more exotic system with planets more massive than Jupiter.”
This research will be published in the Jan. 31 issue of the journal Nature.
Further information: Max Planck Institute for Astronomy press release.
Image: Artist’s impression of the gas and dust disk around the young star TW Hydrae. New measurements using the Herschel space telescope have shown that the mass of the disk is greater than previously thought. Credit: Axel M. Quetz (MPIA)