Three Massive Planets Have Been Discovered in This Bizarre Star System, Baffling Scientists

In an intriguing new discovery, astronomers have identified three huge gas planets orbiting a young star, according to a study published in the Astrophysical Journal Letters. The star already hosted one gas giant—taking the total number of large worlds orbiting it to four.

Not only is this the first time that so many massive planets have been detected around such a young star, but the system has also set a new record for having the most extreme range of known orbits, with the outermost planet more than a thousand times further from the star. Together, these findings raise questions about how such systems formed.

The star, known as CI Tau, is "just" two million years old, meaning it's still close to the beginning of its life cycle. Like other young stars, it is surrounded by a vast disk of dust and ice—known as a protoplanetary disc—in which planets, moons, asteroids and other astronomical objects form.

The CI Tau system—located about 500 light-years away in a highly productive "stellar nursey" region of the galaxy—is already notable for containing the first "hot Jupiter" around such a young star. Hot Jupiters are a class of gas giant exoplanets which orbit extremely close to their host star, which usually means they have incredibly high surface temperatures. The existence of these planets has long puzzled astronomers because they are thought to be too close to their stars to have formed in their current positions.

Using the Atacama Large Millimeter/submillimeter Array (ALMA), the team identified three distinct gaps in the protoplanetary disc, which theoretical modeling suggests were caused by three additional gas giant planets orbiting the star, in addition to the already known hot Jupiter.

These four planets have a range of masses—from the mass of Saturn to 10 times that of Jupiter—and have vastly different orbits. The closest is the hot Jupiter, which is nearer to the star than Mercury is to our Sun. Meanwhile, the farthest two planets orbit at a distance more than three times greater than that of Neptune.

These properties of the system make it particularly puzzling and interesting to astronomers, especially given the fact that that the one percent of stars that host hot Jupiters tend to be 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 disc—would not work in older systems which no longer have a protoplanetary disc," Cathie Clarke from the University of Cambridge's Institute of Astronomy, the study's first author, said in a statement.

It is also unclear whether the three newly detected worlds helped to push the hot Jupiter into its very close orbit and if this process is common among such planets. Furthermore, the researchers have not been able to explain how the outer planets formed.

"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," said Clarke. "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 next steps for the researchers will be to further investigate this puzzling star system at multiple wavelengths using ALMA in an attempt to unravel more of its secrets.