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Surprising molecule helps detect protoplanets

A team of scientists, including Leiden Astronomer Alice Booth, has discovered silicon monosulfide molecules in the dust disk around a young star. Such molecules indicate planet formation. The team made the discovery using the ALMA telescopes. This method provides an alternative when direct observation or imaging of so-called protoplanets is not possible. The team will soon publish the results in The Astrophysical Journal Letters.

The researchers studied the young star HD 169142, in the constellation Sagittarius. The star is surrounded by a large disk of gas and dust that can be seen as a circle from Earth. Over the past decade, several candidate protoplanets have been identified in the disk. A protoplanet is a preliminary stage of a planet where the core has been molten, causing the interior to be differentiated.

In early 2023, astronomers confirmed that candidate HD 169142 b is a giant Jupiter in the making. This was done with the help of imaging from the SPHERE-instrument on the Very Large Telescope, which was also co-developed by the Netherlands.

Chemical clues from the protoplanet

The research team then decided to look into existing ALMA observations to see if they might find chemical clues of the protoplanet in the writing around the star. After all, they suspected that a giant protoplanet like HD 169142 b could cause some violence.

And indeed, in addition to the common carbon monoxide and the rarer sulfur monoxide (SO), the team also detected the silicon monosulfide (SiS) which is not previously observed around young stars.

Artistic rendering of a jupiter-like protoplanet (left) orbiting its star and heating gas forming slilicon monosulfide.

Searching for chemical fingerprints

'SiS came as a surprise,' says lead author Charles Law of the Center for Astrophysics 'Because to capture SiS with ALMA, silicates must be released from dust grains. To do that, huge shock waves must have occurred, caused by gas traveling at high speeds. We suspect that the shock waves were formed by interactions between the giant protoplanet and the surrounding gas.'

Booth adds: 'By looking for chemical fingerprints, we now have a new tool to hunt for planets. We expect to discover more unexpected molecules in the dust disks around young stars in the future.'

Scientific article
SO and SiS emission tracing an embedded planet and compact 12CO and 13CO counterparts in the HD 169142 disk. Door: Charles J. Law, Alice S. Booth & Karin I. Öberg. Accepted for publication in The Astrophysical Journal Letters. [preprint]

This post appeared as a press release on NOVA's website.


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