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SYFY WIRE Bad Astronomy

The weirdest binary: Planet and not-a-star barely orbit each other

By Phil Plait
A rather fanciful piece of art showing the brown dwarf Oph 98A (upper left) orbited by a planetary-mass companion. Separated by 30 billion km, they would each appear as dots from the other. Credit: Thibaut Roger / University of Bern

Astronomers have found a pretty weird binary system about 450 light years from Earth: Neither of its components is a star. Instead, one is a brown dwarf, and the other appears to be a planet orbiting it! Even then, the brown dwarf is on the lower end of things. If it were any less massive it would be a planet itself.

The system is called CFHTWIR-Oph 98, but we'll call it Oph 98 for short. It was found a few years ago in ground-based observations using the Canada-France-Hawaii Telescope's infrared WIRCAM camera (hence the first part of the object's name) in a region of the galaxy where stars are being born which we see in the constellation of Ophiuchus (hence the second name part). It was observed in 2006 and 2012, and identified as a brown dwarf — an object more massive than a planet but too lightweight to ignite sustained nuclear fusion in its core like a star.

Astronomers re-observed it in 2013 with Hubble, and then again in 2020 with the United Kingdom Infrared Telescope. In all three telescope images, a fainter object can be seen very close to the primary (brighter) object, the brown dwarf. Right away they suspected it could be a companion, but how to be sure?

CFHT (left), Hubble (middle) and UKIRT (right) observations of the extremely low-mass object Oph 98 AB (indicated in the middle panel), what is likely a brown dwarf /planet system. Credit: Fontanive et al.

One way is to observe them over several years and track their motion through space. If they move together, they're likely to be connected, but if they don't the faint one is likely to be a background star or galaxy. That's why I mentioned all those observations and their dates: Sure enough, the two are moving together through space, and appear to be physically associated with one another. Therefore we call the primary Oph 98 A and the secondary Oph 98 B (to be honest I think the B should be b since it's a planet, but the naming convention isn't clear in this case since the primary is a brown dwarf).

But that's weird! Given their distance from Earth and their apparent separation in the sky, they must be at least 30 billion kilometers apart. That's a long way; over six times farther than Neptune is from the Sun.

The new observations allowed the astronomers to get a mass estimate for the two objects, and that's where it gets even more fun. Oph 98 A appears to have a mass of about 15 times Jupiter's mass. That's at the very lowest end of the brown dwarf mass range. It likely is able to fuse deuterium (an isotope of hydrogen) in its core, but not for terribly long. After that it will not generate any new energy and just cool off for the next, oh, trillion years or so.

Oph 98 B has a mass of about 8 Jupiters, so it's pretty firmly in the planetary mass range. These numbers are based on the ages of the objects, which the astronomers tag at 3 million years; very young. They may be anywhere from 1–7 million years old, in which case their masses might be somewhat higher or lower. So it's possible that the primary actually is a planet and not a brown dwarf.

A rather fanciful piece of art showing the brown dwarf Oph 98A (upper left) orbited by a planetary-mass companion. Separated by 30 billion km, they would each appear as dots from the other. Credit: Thibaut Roger / University of Bern

Either way, it's a very low-mass system; even added together they are far less massive than the least massive true star. And that's why this is so odd: At a separation of 30 billion kilometers (at least!) they're barely holding on to each other gravitationally. They have what's called the lowest binding energy of any system known; it would only take a small input of energy to unbind them and flung them apart.

Which also raises the question on how they formed at all. Brown dwarfs form in a similar way to stars, where a dense, cold cloud of gas and dust collapses. This creates a swirling disk with the object forming in the center. Planets, though, form from the disk and not from the direct collapse of the cloud.

That can't be the case for Oph 98 B. For one thing, the disk of a brown dwarf doesn't have enough stuff in it to make a planet 8 times Jupiter's mass. For another, the disk is too small to form a planet that far out. It's possible they formed closer together and the secondary got flung out, maybe by an encounter with a passing star, but that seems unlikely in the short time they've been around. Also, they are just barely bound together by gravity, and it's unlikely an encounter would give the planet just that much energy to barely not escape. It's too big a coincidence to swallow.

So it looks like they both formed from direct cloud collapse, and that's a surprise. Scientists love surprises. It means we've learned something, and that there's more to learn.

There are a handful of extremely low-mass binary systems known, but this one has the lowest binding energy known. It's also odd that the primary is about twice the mass of the secondary; in general the two objects tend to have similar masses (that has to do with the formation mechanism which tends to even out the masses).

All of this means there's a lot more to understand about systems like these. They're really faint, which is why we haven't seen many. It helps that this one is so close to us, and so young. That means the two objects are blazing away with the heat of their formation and therefore brighter.

We don't know how they formed, exactly, but we have a decent idea of what their future is. It's very likely they won't last long as a couple; any star that passes reasonably close will yank them apart, and they'll each spend the rest of eternity orbiting the galaxy alone.

Such is the fate of many objects in our Milky Way, including, I'll note, the Sun. But it makes me wonder just how many rogue planets there are out in the black; cold, faint objects essentially invisible to us. There's a lot going on in the galaxy, and we're just now starting to see how much we don't see.