Rogue worlds – planets without homes

Astronomers are finding mysterious planets in isolation

When we think of objects in space we tend to imagine stars or smaller bodies orbiting them. Our theories of planet formation involve stars so perhaps it’s no surprise that we find planets in orbits around their hosts. If astronomers find something sitting indepedently in space, not in orbit around larger bodies, then it’s probably a star. It’s easy to image a star existing in isolation. But sometimes astronomers find weird objects that don’t seem to be stars, yet they aren’t orbiting nearby stars. So what are they?

Three years ago, astronomers found a very weird object in the night sky using the Pan-STARRS1 (PS1) wide-field survey telescope on Haleakala, Maui. They gave it the catchy name PSO J318.6338-22.8603, which you probably shouldn’t add to your baby name inspiration list. It’s 80 light-years from Earth. I like to think Pluto is far from Earth, but it’s less than 0.1% of a light-year away. PSO J318-22 definitely has nothing to do with our solar system. Surprisingly, it doesn’t seem to be part of any solar system.

Image © N. Metcalfe & Pan-STARRS 1 Science Consortium

The mystery object was big enough to be seen by the telescope (it has a mass 8.3 times that of Jupiter) so we’re talking about a large body such as a star or a planet. It isn’t closely orbiting a star so it makes sense to assume it’s a star itself. The initial hypothesis was that it was a failed star where nuclear fusion never took hold. We’ve recently talked about nuclear fusion, how it fuels our own Sun, and how we’re on the verge of unlocking that power for ourselves. Astronomers were leaning towards a stellar object because it was quite hot, but it really should be much brighter. This led the astronomers to form a new hypotheses: a rogue planet.

Artist's impression of PSO J318.5-22. Image © MPIA/V. Ch. Quetz

Finding planets outside our own solar system is nothing new. However, most planets we discover are orbiting star systems. Indeed, our theories of how planets form involve stars so it’s no surprise that we find planets orbiting their hosts. That’s what makes this object quite mysterious; it appears to be floating through space alone and not part of a solar system. A new study has confirmed that the object is indeed a rogue planet and not a brown dwarf star. Being part of the Beta Pictoris moving group means that astronomers can estimate the planet’s age. At only 23 million years old, J318.5-22 is too young to be a brown dwarf. It’s also far too cold. We’re left with a large planet without a solar system to call home.

It’s hard to imagine how such a young planet can be so isolated but discoveries of other rogue planets might provide clues. A potential rogue planet was discovered 8 years ago and given another nice name: 2MASS J2126-8140 (we’ll call it 2MASS J2126). It’s a weird gas giant that’s thought to have clouds of molten iron (bring an umbrella). A new study has claimed that 2MASS J2126 isn’t completely independent in space; it’s just really, really far away from its host star. If true, this is the biggest solar system we’ve ever discovered.

The planet 2MASS J2126 and its star, TYC 9486-927-1. The arrow indicates where both objects will be found 1000 years from now. Notice they travel together. Image © 2MASS/S Murphy/ANU

The authors of the new study found that 2MASS J2126 is moving through space along with a star known as TYC 9486-927-1. Both objects are separated by 600 billion miles, which is 175 times further than the distance between Pluto and our own Sun, so you can see why astronomers believed the planet was alone out there. It’s so far from its host star that a single orbit (that takes our planet 365 days) would take just under 1 million years. That’s a long work day. Nobody knows how such a big planetary system can form or how it could survive at that scale. What interests me is that it might begin to explain where other truly rogue planets come from.

Artist's impression of 2MASS J2126. Image © University of Hertfordshire/Neil Cook

Think back to PSO J318.5-22; it doesn’t appear to be orbiting a star. It’s as rogue as they come, though on a large scale it is part of a group of young stars moving across the night sky. How did it come to be so lonely? One possibility is that it formed by planetary accretion around a host star, as most planets are thought to, but some event knocked it far beyond its original orbit soon after it was formed. Perhaps the extremely distant orbit of 2MASS J2126 is some sort of an intermediate, still clinging onto its host star and not fully rogue. This seems plausible but I’m yet to be convinced because of how young the stars are in the Beta Pictoris region. Is 23 million years enough time to get that far from the host star?

The other possibility, which is perhaps more exciting, is that PSO J318.5-22 was formed by processes we know nothing about. Maybe it was always alone. We’re still learning about planetary formation and it’s a big universe out there. Perhaps this bizarre world represents a type of planet formation we’re totally unaware of.

This is what excites me about astronomy; we’ve learned a lot over the centuries but so much remains unknown. It’s not a hopeless sense of the unknown though, as we’re now creating telescopes bigger than ever before and making discoveries that let us probe the cosmos in entirely new ways. It’s a great time to be interested in how the universe works.


Main image ©  ESO/L. Calçada/P. Delorme/R. Saito/VVV Consortium