What if we could ‘directly’ weigh a planet-forming disk without even visiting one? A group of astronomers from Japan have found a new technique to do so and it turns out to be the best method of all.

When young stars form, they are often followed by a disk of gas and dust that orbits around them, known as the protoplanetary disk. Planets, like the ones we have in our Solar System, usually form in these regions. Radio waves are the main footprints of these disks, and by studying the spectra of a disk, astronomers can understand a lot about what they are made of and how planets really form. 

One of the main ingredients of a protoplanetary disk is Hydrogen gas (H₂). But this gas doesn't beam strong radio waves making it very difficult to directly measure how much Hydrogen a disk has. So astronomers instead measure the second most abundant gas, Carbon monoxide (CO), as a “substitute” or a proxy for Hydrogen. Recent studies showed, however, that this method is not as precise as scientists needed it to be. 

So a team in Japan came up with a different idea. Together with his colleagues, Tomohiro Yoshida, from the University for Advanced Studies in Japan, began to study the "closest" protoplanetary disk we know about. The disk, TW Hya, is around TW Hydrae, a star 196 light-years away from Earth (almost 50 times farther than Alpha Centauri!) in the constellation Hydra - the Sea Serpent. 

Using data from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, Yoshida and his team got a very detailed picture of the disk in radio wavelengths, 15 times more sensitive than in previous studies. This allowed the team to not only analyse the wavelengths of different ingredients in the disk, like CO, but also its shape.

From the shape of the CO spectra, the team could now directly measure more detailed properties of the gas such as its pressure and total mass, without having to make any assumptions like before. With this new technique, Yoshida and team will now investigate different types of planet-forming disks to understand the process in detail.
 

Image: Observational image of the protoplanetary disk around TW Hydrae showing the distributions of solid particles (red), carbon monoxide (blue), and dense gas (white). (Credit: T. Yoshida, T. Tsukagoshi et al. - ALMA (ESO/NAOJ/NRAO))