Season 17, Episode 52: Rogue Planets Explained

Hey StarTalkians! Episode 52 of season 17 was a Cosmic Queries edition, featuring Neil, Chuck and guest Professor David Kipping discussing all things planetary. About half way through the podcast, Chuck set the ball rolling on an interesting discussion about wandering “rogue planets”:

Rogue Planets & Exomoons with David Kipping - StarTalk Radio

(from 31:30)

There are a lot of threads that could be picked up there – the history of our own solar system, for one – but I left mainly wanting to know more about the search for rogue planets. So here’s a quick primer.

How Planets Go Rogue

Although there are several ways planets can become rogue, the simplest one is outlined in a paper from the 90s focusing on two Jupiter-sized planets.

If the planets evolved at different rates, or if they continue gaining mass, it can lead to instability in their orbits and interactions between them. The authors found that when they didn’t collide (about 50% of cases), one planet tended to be ejected and become a rogue, while the other settled into a tighter, more squashed-out orbit. When they also included smaller planets, these often got ejected from the system too.

How We Find Rogue Planets

Microlensing is a common approach to finding these rogues, as discussed on the podcast. Gravitational lensing happens on small scales too, and sensitive-enough detectors – as the Nancy Grace Roman Space Telescope will have – can pick up lensing caused by smaller objects.

Even though rogue planets don’t give off light, their gravity does alter the light profile from stars. Most of the work on rogue planets has involved careful observation of a star for the characteristic bump in brightness from microlensing.

Low Mass Rogues

The challenge is spotting this small effect, because for a rogue planet, it won’t repeat. In fact, the length of time this goes on for is a key metric for inferring the size of the object. A 2020 paper described the (not cool-y named) OGLE-2016-BLG-1928 event, which only lasted for about 41 and a half minutes. The main result (see images) shows just how hard this is to catch. This object is assumed to be near Mars’ mass.

How Many Rogue Planets Are There?

One recent estimate suggests that there are about 20 rogue planets per star in our galaxy, and that the Roman space telescope will find something around 1,000 of them.