Puzzles from an Ensemble of Young Planets

Blistering hot, giant planets zip around many stars similar to the Sun. New observations of one such planet — likely surrounded by a set of three gas-giant siblings — is now raising questions about the formation of giant planets.

Birthing a Hot Jupiter

Hot Jupiters — gas giants orbiting their hosts at radii of < 0.1 AU — are thought to orbit around 1% of main-sequence solar-type stars. In spite of the many hot Jupiters we’ve discovered, however, we still don’t fully understand how these toasty giants form. Are they born and evolve in situ, alongside their host? Or do they develop further out in the early protoplanetary disk of gas and dust surrounding a young star, and migrate inward later in their lifetimes? What role might outer gas-giant siblings play in this process?

One challenge to answering these questions is we’ve primarily observed older hot Jupiters, rather than those still in the process of forming. But the protoplanetary disk surrounding the young star CI Tau hosts a nascent hot Jupiter — and new observations reveal evidence for additional gas giants in the disk. What can we learn from this system?

A Unique Disk

The Atacama Large Millimeter/submillimeter Array (ALMA) is progressively building its reputation as an imager of the detailed structure within young protoplanetary disks; you might recall one of its first releases, the spectacular image of the concentric gaps and rings of the disk surrounding HL Tau.

In a new study led by Cathie Clarke (University of Cambridge’s Institute of Astronomy, UK), this groundbreaking telescope has been used to image another young star system, CI Tau, which hosts the first hot-Jupiter candidate found still within a protoplanetary disk.

Though CI Tau is roughly the same mass and luminosity as the Sun, it’s only about 2 million years old. The presence of the 11.3-Jupiter-mass giant planet in a close-in orbit in the disk therefore shows that whatever the formation mechanism for hot Jupiters, they must arrive at their close orbits very rapidly!

New Planets, New Challenges

The hot Jupiter isn’t CI Tau’s only source of intrigue, though: Clarke and collaborators demonstrate that this disk likely hosts another three gas-giant planets of 0.75, 0.15, and 0.4 Jupiter masses, orbiting at 14, 43, and 108 AU. The authors reach this conclusion by fitting detailed dynamical models of the dust and gas to the ALMA data as well as a wealth of supplementary data for the well-studied system.

If confirmed, the presence of these outer planets poses a significant challenge to our understanding of planet formation. Based on current models, the outermost planets shouldn’t have been able to accrete enough material from the outer disk in just 2 million years to reach their current sizes. And even if this mystery could be explained, another exists: the planets’ growth past a certain point should have triggered rapid inward migration.

Though the new observations of CI Tau have raised as many questions as answers, they solidify the association between close-in hot Jupiters and gas giants on wider orbits. As ALMA continues to build a census of young protoplanetary disks, we can hope that future observations will shed further light on the process of building giant planets.

Citation

“High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 au,” C. J. Clarke et al 2018 ApJL 866 L6. doi:10.3847/2041-8213/aae36b