Astronomers Just Spotted What Might Be a Hint of a Hidden Planet in Star System HR 8799

An unprecedented image of HR 8799, a star system 129 light-years away, has revealed a mysterious disc of material floating around its edge. The new image could explain the strange behavior of the planets in that system. And it could mean there's another planet lurking there that we haven't spotted yet.

HR 8799 has some of the only exoplanets we have ever viewed directly from a telescope on Earth. Direct imaging is really hard because exoplanets are so far away. Spotting these four planets was the equivalent of someone in New York City looking at a lighthouse in California and trying to find a few fireflies in front of that lighthouse, according to Steve Jefferson, spokesman for the Keck Observatory in Hawaii.

Here's what the direct image of the four planets looks like:

Now, using the Atacama Large Millimeter/submillimeter Array radio observatory in Chile, astronomers have gotten an even clearer image of the star system, and it's revealed a huge disc of dust and comets orbiting on the outer boundary. The yellow inset shows the star and its planets, while the disc is shown in blue:

"These data really allow us to see the inner edge of this disk for the first time," Mark Booth, lead author of the study, explained in a statement. "By studying the interactions between the planets and the disk, this new observation shows that either the planets that we see have had different orbits in the past or there is at least one more planet in the system that is too small to have been detected."

So the irregular shape of the disc means that the planets' orbital path has changed over time, or there's a fifth planet hiding somewhere. Best of all, we have enough data on this star system to compare it to our own.

"This is the very first time that a multi-planet system with orbiting dust is imaged, allowing for direct comparison with the formation and dynamics of our own solar system," said Antonio Hales, co-author of the study.