Rogue planets: How wandering bodies in interstellar space ended up on their own

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By Joanna Barstow for The Conversation

We now know about 5,000 planets outside the solar system. If you were to imagine what it would look like on one of these distant worlds, or exoplanets, your mental picture would likely include a parent star — or more than one, especially if you’re a Star Wars fan.

But scientists have recently discovered that more planets than we thought are floating in space alone – unlit by a friendly stellar companion. These are icy “floating planets,” or FFPs. But how did they end up on their own and what can they tell us about the formation of these planets?

Finding more and more exoplanets to study has, predictably, broadened our understanding of what a planet is. In particular, the line between planets and “brown dwarfs” — cool stars that cannot fuse hydrogen like other stars — has become increasingly blurred.

What determines whether an object is a planet or a brown dwarf has long been debated – is it a matter of mass? Do objects cease to be planets if they undergo nuclear fusion? Or is the way the object was formed more important?

While about half of stars and brown dwarfs exist in isolation, with the rest being in multiple star systems, we generally think of planets as subordinate objects orbiting a star.

More recently, however, improvements in telescope technology have allowed us to see smaller, cooler isolated objects in space, including FFPs – objects that have too low a mass or temperature to be considered brown dwarfs.

What we still don’t know exactly is how these objects formed. Stars and brown dwarfs form when a region of dust and gas in space begins to fall on itself.

This region becomes denser, so that more and more matter falls on it (due to gravity) in a process called gravitational collapse.

Eventually, this ball of gas becomes dense and hot enough for nuclear fusion to start – hydrogen burns up in the case of stars, deuterium (a type of hydrogen with an extra particle, a neutron, in the nucleus) burns up for brown dwarfs.

FFPs can form in the same way, but never grow large enough for fusion to begin. It is also possible that such a planet begins its life in orbit around a star, but at some point is expelled into interstellar space.

How to Spot a Wandering Planet

Rebellious planets are difficult to spot because they are relatively small and cold. Their only internal heat source is the energy left over from the collapse that caused them to form. The smaller the planet, the faster the heat will be removed.

Cold objects in space emit less light, and the light they emit is redder. A star like the Sun has its emission peak in the visible; the peak for an FFP is rather in the infrared.

Because it is difficult to see them directly, many such planets have been discovered using the indirect method of “gravitational microlensing”, when a distant star is in the correct position for its light to be gravitationally distorted by the FFP.

However, detecting planets via a single, one-time event has the disadvantage that we will never be able to observe that planet again. We also don’t see the planet in context with its surroundings, so we are missing vital information.

To observe FFPs directly, the best strategy is to catch them when they are young. This means that there is still a reasonable amount of heat left over from their formation, so they are the brightest. In the recent study, the researchers did just that.

The team combined images from a large number of telescopes to find the faintest objects within a group of young stars, in a region called Upper Scorpius.

They used data from large general-purpose surveys combined with their own more recent observations to generate detailed visible and infrared maps of the sky area covering a 20-year period. They then looked for faint objects moving in a way that indicated they were members of the star cluster (rather than much more distant background stars).

The group found between 70 and 170 FFPs in the Upper Scorpion region, making their sample the largest directly identified to date – although the number has significant uncertainty.

Rejected planets

Based on our current understanding of gravitational collapse, there appears to be too many FFPs in this group of stars for them all to have formed this way. The study authors conclude that at least 10% of them must have started life as part of a star system, forming in a disk of dust and dust around a young star rather than by gravitational collapse. At some point, however, a planet could be ejected due to interactions with other planets. In fact, the authors suggest that these “rejected” planets might be just as common as planets that have been alone all along.

If you’re freaking out about the Earth’s sudden rotation in deep space, you probably don’t have to worry – these events are much more likely early in the formation of a planetary system when there are has many planets jostling for position. . But it’s not impossible – if something outside of an established planetary system, like another star, were to disturb it, then a planet could still be detached from its sunny home.

While we still have a long way to go to fully understand these wandering planets, studies like this are valuable. Planets may be revisited for deeper and more detailed investigation as new telescope technology becomes available, which may reveal more about the origins of these strange worlds.

(The author is Ernest Rutherford Fellow of the Open University)

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