Do spiral protoplanetary discs mean exoplanets in orbit?

The Sky at Night's Chris Lintott considers the spiral features found in the dusty discs around young stars, and what it might mean for our understanding of planetary formation.

Following clues: a computer model of the spiral arms in the disc around the hot star MWC 758. Credit: ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University)

Almost every week, clutches of new exoplanets are found around stars near and far. Yet the real progress in understanding planetary systems may come from the quiet revolution in the study of the discs of dust and gas from which planets form. Studying these discs is allowing us to unpick the details of how forming planets move, grow and interact.

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The most intriguing features are the spiral arms which have been spotted in several discs.

The spiral arms in galaxies like the Milky Way are features sculpted by gravity, but most protoplanetary discs are not massive enough for gravity to play this kind of role.

Instead, it’s thought that spirals are the result of the disc being stirred up by a forming planet, meaning that we can use them to guide us to place where planet formation is happening.

A paper entitled Thermal Infrared Imaging of MWC 758 with the Large Binocular Telescope: Planetary Driven Spiral Arms? by Kevin Wagner et al. studies just such a disc around the hot star MWC 758 in Taurus, which has been shown to play host to a couple of spiral arms.

The system has been simulated by theorists in a computer, an effort that suggested a massive exoplanet at least five times Jupiter’s mass at the end of one of the arms could explain the spiral.

But is there really such an exoplanet? The team set out to use the Large Binocular Telescope in Arizona, a magnificent beast with two 8-metre scopes that is good at observing in the broad infrared: perfect for looking at warm dust and the exoplanet it might enfold.

The resulting images are great, showing spiral arms in the disc more clearly than ever.

The two arms are not symmetrical, with one short arm and another looser, long one.

This lack of symmetry seems to suggest a system in flux, rather like the arms of the Whirlpool Galaxy, M51, stretching out towards a perturbing companion.

Observational data of the disc around MWC 758. Credit: NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University)
Observational data of the disc around MWC 758. Credit: NASA, ESA, ESO, M. Benisty et al. (University of Grenoble), R. Dong (Lawrence Berkeley National Laboratory), and Z. Zhu (Princeton University)

And there does seem to be such a feature here – a blob that the team call CC1 (‘Companion Candidate’) sits just off the end of the longer arm.

Is this the planet we were looking for? It’s in the right place and it seems to be bright enough to represent a planet between two and five times the mass of Jupiter, and perhaps even bigger – large enough to disturb the disc sufficiently to create the observed arms.

The really important clue is that CC1 is red – it is, in fact, only detected at longer wavelengths.

Not only is this what we’d expect for a warm planet, it is different from the rest of the disc, so it’s not likely to be just a poorly imaged extended bit of spiral arm.

The team plan to keep monitoring the system and watching how the features move over the next decade or so. Already, we can see what might be a large planet in the process of formation – an amazing triumph of modern astronomical instruments and ingenuity.

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Chris Lintott is a professor of astrophysics at the University of Oxford and a co-host of The Sky at Night. He was reading Thermal Infrared Imaging of MWC 758 with the Large Binocular Telescope: Planetary Driven Spiral Arms? by Kevin Wagner et al. Read it online at https://arxiv.org/abs/1907.06655