Giant planets around young star raise questions about how planets form

High-resolution millimeter continuum imaging of the disk surrounding the young star CI Tau, a system hosting the first hot Jupiter.

Artist's impression of CI Tau Credit: Amanda Smith, Institute of Astronomy
Artist's impression of CI Tau Credit: Amanda Smith, Institute of Astronomy

Specialists have distinguished a young star with four Jupiter and Saturn-sized planets orbiting around it. It is the first event when that such a large number of massive planets have been identified in such a young system.

The system has likewise set another record for the most extraordinary scope of orbits ever observed: the peripheral planet is in excess of a thousand times assist from the star than the innermost one, which brings up fascinating issues about how such a system may have shaped.

Exploring more, scientists found that the star is just two million years old – a ‘toddler’ in astronomical terms – and is surrounded by a huge disc of dust and ice. This disc, known as a protoplanetary disc, is where the planets, moons, asteroids and other astronomical objects in stellar systems form.

The star, CI Tau, is located about 500 light years away in a highly-productive stellar ‘nursery’ region of the galaxy. Its four planets differ greatly in their orbits: the closest (the hot Jupiter) 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.

Although, the star is known for consisting hot-Jupiter revolving around it. The 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, astronomers at the University of Cambridge have used the Atacama Large Millimeter/submillimeter Array (ALMA) to search for the siblings of this hot Jupiter. Their image uncovered three particular holes in the disc, which, as per their hypothetical displaying, were no doubt caused by three extra gas giant planets additionally orbiting the star.

This discovery raises many questions for astronomers. Around 1% of stars host hot Jupiters, but most of the known hot Jupiters are hundreds of times older than CI Tau.

Professor Cathie Clarke from Cambridge’s Institute of Astronomy, the study’s first author said, “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 disc – would not work in older systems which no longer have a protoplanetary disc.”

However, it is still 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.

Clarke said, “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. 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.”

Astronomers are further planning to study this puzzling system at multiple wavelengths to get more clues about the properties of the disc and its planets. In the meantime, ALMA – the first telescope with the capability of imaging planets in the making – will likely throw out further surprises in other systems, re-shaping our picture of how planetary systems form.

The results are reported in The Astrophysical Journal Letters.