Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have discovered a 2-million-year-old planetary system containing at least four Saturn- to Jupiter-sized gas giants, orbiting the T Tau-type star CI Tau. This is the first time that so many massive exoplanets have been detected in such a young planetary system. The CI Tau system has also set a new record for the most extreme range of orbits yet observed.
An artist’s impression of four gas giants in orbit around CI Tau. Image credit: University of Cambridge.
CI Tau is located 518 light-years away in a highly-productive stellar ‘nursery’ region and is surrounded by a huge protoplanetary disk.
The star was already known to be remarkable because it contains the first so-called hot-Jupiter exoplanet, CI Tau b, to have been discovered around such a young star.
Although hot Jupiters were the first type of exoplanet to be discovered, their existence has long puzzled astronomers because they are often thought to be too close to their parent stars to have formed in situ.
Now, Professor Cathie Clarke from the University of Cambridge Institute of Astronomy and co-authors have used ALMA to search for CI Tau b’s planetary siblings.
Their observations revealed three distinct gaps in the disk, which, according to their theoretical modeling, were most likely caused by three additional gas giants also orbiting the young star.
CI Tau’s four planets differ greatly in their orbits: the closest — CI Tau b — is within the equivalent of the orbit of Mercury, while the farthest orbits at a distance more than three times greater than that of Neptune.
The two outer planets are about the mass of Saturn, while the two inner planets are respectively around one and 10 times the mass of Jupiter.
“The discovery raises many questions,” the astronomers said.
“Around 1% of stars host hot Jupiters, but most of the known hot Jupiters are hundreds of times older than CI Tau.”
“It is currently impossible to say whether the extreme planetary architecture seen in CI Tau is common in hot Jupiter systems because the way that these sibling planets were detected — through their effect on the protoplanetary disk — would not work in older systems which no longer have a protoplanetary disk,” Professor Clarke said.
It is also unclear whether the sibling planets played a role in driving the innermost planet into its ultra-close orbit, and whether this is a mechanism that works in making hot Jupiters in general. And a further mystery is how the outer two planets formed at all.
“Planet formation models tend to focus on being able to make the types of planets that have been observed already, so new discoveries don’t necessarily fit the models,” Professor Clarke said.
“Saturn mass planets are supposed to form by first accumulating a solid core and then pulling in a layer of gas on top, but these processes are supposed to be very slow at large distances from the star.”
“Most models will struggle to make planets of this mass at this distance.”
The discovery is reported in the Astrophysical Journal Letters.
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C.J. Clarke et al. 2018. High-resolution Millimeter Imaging of the CI Tau Protoplanetary Disk: A Massive Ensemble of Protoplanets from 0.1 to 100 AU. ApJL 866 (1); doi: 10.3847/2041-8213/aae36b
