The Rogue Planets of Orion

All the lonely planets, where do they all come from?

Jul 10, 2024

Young stars shine amidst pillars of gas in the Orion Nebula. Image credit: NASA, ESA, M. Robberto ( Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team

Long ago, the story goes, our solar system had a fifth giant planet. In form this world may have looked like Neptune - that's to say, it was an ice giant, bluish in colour and held perhaps twenty times the mass of Earth. It probably lay somewhere between Saturn and Uranus, though back then the gas giants orbited much closer to the Sun, and the outer planets might have been arranged in a different pattern.

At some point Saturn and Jupiter suddenly swung outwards. Their movements pushed Uranus and Neptune outwards too, sending them careening through the belt of ice and dust at the solar system’s edge. Scattered comets rained inwards, bombarding the inner planets and leaving scars, still visible today, on the Moon.

Somewhere amidst this chaos the fifth planet was lost. Perhaps, some simulations hint, it first moved inwards and then crossed the orbits of both Jupiter and Saturn. For this boldness it was expelled and pushed so far out into space that it escaped the grasp of the Sun entirely. Afterwards it was left to wander the galaxy alone: a rogue planet, shorn of its star.

The truth of this story is debatable. We have no direct evidence of such a world, of course, and can only trace its presence by running the orbits of the planets backwards. And if we do suppose it really existed, then this world was lost some four billion years ago. We have no chance of finding it again. Even if we somehow divined where it lies, the planet is surely too distant, too dark and too cold to ever be seen again.

The point, however, remains. Planets can get lost, and when they do they drift alone, spinning onward through an uncaring cosmos. Most are cold and dark places, heated only by whatever internal warmth they retain from birth. Few are easy to see. Of the millions that must exist, telescopes have spotted a mere handful, and traced a few dozen more through their subtle gravitational fingerprints.

The Orion Nebula

Still, if we’re going to find rogue planets anywhere, then the Orion Nebula seems like a promising place to start. For centuries it has been enticing astronomers with its beauty, drawing the gaze of ancient observers and modern observatories alike. Though it lies more than a thousand light years from Earth, it is bright, shining strongly enough to be visible with the naked eye.

All that light is coming from a dense cluster of stars, counting several thousand in total and many of which formed in the past few millennia. Around them, in all likelihood, are newborn planets which, since they were forged in fire, are hot and glowing brightly in infrared light. That makes them easy targets for observatories like the James Webb Space Telescope.

In the autumn of 2022, a pair of European astronomers directed that telescope to peer into the densest part of the Orion Nebula. What they saw surprised them. Hundreds of rogue planets appeared, measuring from half the mass of Jupiter to more than thirteen times greater. Even more surprisingly, several dozen of those planets appeared to be orbiting one another; forming binary pairs in a way that had never been expected.

That makes the origin of these planets mysterious. Had they appeared as individuals, they could have been explained as lost worlds, expelled from their star systems. Yet if this were indeed the case, it is unlikely so many would have ended up in pairs. Were these instead failed stars, the researchers speculated? Could it be that these odd planets had formed like stars, but failed to achieve the mass necessary to ignite nuclear fusion?

We do know of objects like this, which astronomers often call brown dwarfs. Yet brown dwarfs tend to be much larger, and the fact that some of these planets were smaller than Jupiter raises doubts that they could have formed in the same way. And even if this puzzle can be overcome, there seem to be many more of them in binary pairs than should be possible.

These rogue planets are, in other words, something unexpected: something unexplained by our current ideas about star and planet formation, and about how planets can sometimes get lost. They imply that something is missing from those ideas, and that there might be many more Jupiter sized rogue planets out there than we had ever imagined. Recognising this fact, the researchers dubbed the objects JUMBOs - Jupiter Mass Binary Objects - and concluded they might have formed in a way quite different to either stars or planets.

The JUMBOs

What we know about these JUMBOs is so far limited. The observations show they are young and large objects, either similar in mass to Jupiter or up to a dozen times larger. They come in pairs, although the connections between them are weak: the giant planets orbit one another at distances of billions of miles, far more distant in most cases than the Sun and Pluto.

Some care, however, should be taken here. The JUMBOs we have seen are young planets, likely no more than a million or so years old. We may not be able to see older ones: rogue planets cool over time, and that means they shine ever less brightly. If there are older JUMBOs out there, they may just not be visible to our telescopes.

That said, the JUMBOs do seem to be bright. In follow-up studies, astronomers examined the Orion Nebula with a radio telescope. Some of the JUMBOs, they found, are also visible in radio frequencies; which implies they are giving out a lot of energy. When the astronomers revisited old surveys of the same region, they found signs that the JUMBOs are moving slowly, and so are missing the kick of speed that being expelled from a star system might have given them.

That, along with the binary nature of these planets, makes it unlikely they are true rogue planets lost from their stars. Instead, astronomers are now thinking, these worlds could have formed independently of stars. The obvious way this could have happened is through gravitational collapse, a process in which a cloud of gas suddenly collapses to form a star.

Yet for this to happen, the cloud must be above a critical level of temperature and density. The clouds that meet this criteria typically collapse into larger objects, most of which are dozens of times the mass of Jupiter. Something weighing in at just a single Jupiter mass should only rarely emerge from this process, unless we are missing something fundamental about how it works.

There are some plausible options for this, as recent papers have outlined. But possibly, and perhaps more likely, there is a third way - one in which star formation ends early as a system is blasted by radiation from another nearby star. Since stars often form in clusters, it is possible that the smaller or later members of those clusters find themselves losing mass faster than they can gain it. In this case they would end up in a kind of failed state, in which fusion never begins and they end up as vast hot planets instead.

A fourth option is simpler, but far less exciting. The Orion Nebula is dusty, and that dust might be scattering the light coming from stars. If so, those stars might appear redder than normal, and so more like planets than stars. If that is the case, then the JUMBOs are merely an illusion - and this, to be blunt, would not be the first time astronomers have been fooled by cosmic dust.

If the JUMBOs are real though - and the researchers who discovered them insist they are - then they hold interesting implications for the number of rogue planets. There might well be millions more of them out there than astronomers had ever imagined. And that would reshape our view of what lies in the vast darkness between the stars.