Baby planets form in disks surrounding young stars, but the details of this process remain unclear - especially because the planets are often blanketed with dusty gas, hiding them from view. Massive gas planets like Jupiter and Saturn are thought to form by accreting gas onto rocky cores that gradually carve out lanes in the disk.
How can we tell if accretion is happening in a protoplanetary disk? As growing planets collect gas and dust, they also launch material into their surroundings in the form of outflows. As outflowing gas pummels its surroundings, shocks form, triggering the formation of molecules like sulfur monoxide (SO). That gives researchers an in - the planet might be hidden, but emission from these shock-formed molecules can announce its position.
An Archival Search
This tells us how to potentially find baby planets, but where to look? One of the best places to search for signs of planet formation is around TW Hydrae, an 8-million-year-old star less than 200 light-years away. TW Hydrae possesses the nearest known protoplanetary disk, which from our vantage point appears nearly face on, with concentric light and dark rings like a bullseye. Researchers previously found two gaps in this disk, at 26 and 42 au, that could be explained by two roughly 4-Earth-mass planets. In addition, a clump of emission at 52 au hinted at the presence of a circumplanetary disk feeding gas to a growing planet.
Tomohiro Yoshida (National Astronomical Observatory of Japan) and collaborators analyzed archival data from the Atacama Large Millimeter/submillimeter Array (ALMA) to search for signs of outflows from a baby planet in the TW Hydrae disk. The team spotted an arc of emission from SO molecules originating from a gap 42 au from the star - exactly where a planet is purported to be.
Shocking Evidence
What does modeling say about the origin of this emission? The authors used ballistic outflow modeling to show that the SO outflow could be explained by a growing planet with a mass of 4 Earth masses. Combining estimates of the mass-accretion and mass-loss rates, the team finds an overall rate for the growth of the planet that matches theoretical expectations for a 4-Earth-mass planet.
With evidence for outflows already in hand, Yoshida's team plans to continue the search, conducting further observations to look for evidence of the outflow in emission from other promising molecules, like silicon monosulfide. Overall, this work solidifies another line of evidence for the presence of a planet in the 42-au gap of TW Hydrae, and we can expect future observations to illuminate this growing planetary family further!
Research Report:Outflow Driven by a Protoplanet Embedded in the TW Hya Disk
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