Giant planets surrounding a young star raise questions about how planets form

Researchers have identified a young star with four planets the size of Jupiter and Saturn orbiting it, the first time that many gigantic planets have been detected in such a young system. The system has also set a new record for the most extreme range of orbits ever observed: the farthest planet is more than a thousand times farther from the star than the innermost, raising questions interesting about the formation of such a system.

The star is only two million years old – a toddler in the astronomical sense – and is surrounded by a huge disk of dust and ice. This disk, called protoplanetary disk, is the place of formation of planets, moons, asteroids and other astronomical objects in star systems.

We already knew that the star was remarkable because it contains the first so-called Jupiter, a gigantic planet in orbit very close to its parent star, which was discovered around a star so young. Although the Jupiters were the first type of exoplanets 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.

Today, a team of researchers led by the University of Cambridge has used Atacama's ALM (Large Millimeter / Submilleter Array) system to search for planetary "siblings" related to this young Jupiter. . Their image revealed three distinct gaps in the disc, which, according to their theoretical modeling, were probably caused by three other giant gas planets also gravitating around the young star. Their results are reported in the Astrophysical Journal Letters.

The star, CI Tau, is located about 500 light-years away from the galaxy's highly productive stellar "nursery". Its four planets differ greatly in their orbits: the closest (the hot Jupiter) is in the equivalent of the orbit of Mercury, while the farthest orbit at a distance more than three times greater than that of Neptune. The two outer planets correspond roughly to the mass of Saturn, while the two inner planets measure about 1 and 10 times the mass of Jupiter, respectively.

The discovery raises many questions for astronomers. About 1% of the stars host hot Jupiters, but most of the hot Jupiters are hundreds of times older than CI Tau. "It is currently impossible to say whether the extreme planetary architecture observed in the Tau CI is common in hot Jupiter systems, because the way these twin planets were detected – by their effect on the protoplanetary disk – would not work in older systems that no longer have a protoplanetary disk, "said Professor Cathie Clarke of the Cambridge Astronomical Institute, first author of the study.

According to the researchers, it is also unclear whether twin planets played a role in the trajectory of the farthest planet in its near-Earth orbit and whether this mechanism works to make Jupiters warmer in general. Another mystery lies in the formation of the two outer planets.

"Planet formation models tend to focus on the ability to create the types of planets already observed, so new discoveries do not necessarily match the models," Clarke said. "Saturn's mass planets are thought to be formed by first accumulating a solid core and then covering a layer of gas, but these processes are supposed to be very slow at a great distance from the star. bad to make planets, mass at this distance. "

The task ahead will be to study this complex system at several wavelengths to obtain more clues about the properties of the disc and its planets. Meanwhile, ALMA – the first telescope capable of creating images of planets – is likely to sow other surprises in other systems, re-shaping our image of how planetary systems are formed.

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More information:
C. J. Clarke et al. High resolution millimeter imaging of the CI Tau protoplanetary disk: a massive set of protoplanets from 0.1 to 100 ua, The astrophysical journal (2018). DOI: 10.3847 / 2041-8213 / aae36b