The article was originally published at The Conversation.
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There are almost 5,000 planets outside the solar system. If you were to picture what it would be like on one of these distant worlds, you would probably include a parent star or more than one.
Scientists recently discovered that more planets than we thought were floating through space on their own. These are free-floating planets or FFPs. What can they tell us about how planets form?
The 10 biggest discoveries of the year.
Finding more and more exoplanets to study has widened our understanding of what a planet is. The line between planets and brown dwarfs has become blurred. Is it a question of mass if an object is a planet or a brown dwarf? Nuclear fusion may cause objects to cease to be planets. Is the way in which the object was formed the most important?
We usually think of planets as objects in the vicinity of a star, since half of stars and brown dwarfs are in isolation. Improvements in telescope technology have allowed us to see smaller and cooler isolated objects in space, including FFPs, which are objects that have too low a mass or temperature to be considered brown dwarfs.
We don't know how these objects formed. Stars and brown dwarfs form when a region of dust and gas falls in on itself. The region becomes denser and more material falls onto it in a process called gravity collapse.
This ball of gas becomes dense and hot enough for hydrogen to be burned in the stars, deuterium, in the nucleus, in order to start nuclear fusion. FFPs form in the same way, but never get big enough for fusion to start. It is1-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-65561-6556
Small and cold, rogue planets are hard to spot. Remaining energy left over from the collapse is their only source of internal heat. The smaller the planet, the quicker the heat will be dissipated.
Cold objects in space emit less light. The peak of a star in the visible range is different from the peak of a FFP in the IR. Because it is difficult to see them directly, many such planets have been found using the indirect method ofgravitational microlensing.
The most mysterious worlds are being hunted by rogue planets.
The disadvantage of detecting planets via a single, unique event is that we cannot observe them again. We don't see the planet in context with its surroundings, so we don't have vital information.
Catching FFPs while they are young is the best way to observe them. They are at their best because there is still a lot of heat left over from their formation. Researchers did that in the recent study.
In order to find the faintest objects within a group of young stars, the team combined images from a large number of telescopes.
They used data from large, general purpose surveys combined with more recent observations of their own to generate detailed visible and infrared maps of the area of sky over a 20-year period. They looked for faint objects moving in a way that indicated they were members of the group of stars.
The group found between 70 and 170 FFPs in the Upper Scorpius region, which is the largest directly identified sample so far.
There seems to be too many FFPs in this group of stars for them all to have formed in that way. At least 10% of them must have started out life in a disk of dust around a young star, according to the study authors. A planet could get ejected due to interactions with other planets. The authors suggest that the planets that have been alone from the beginning are just as common as the planets that have beenjected.
When there are a lot of planets for position early on in the formation of a planetary system, you don't need to worry about Earth suddenly spinning off into the depths of space. It is possible for a planet to be detached from its home if something external to an established planetary system were to disrupt it.
We still have a long way to go to fully understand these planets, but studies like this one are valuable. As new telescope technology becomes available, the planets can be reexamined for further investigation, which may reveal more about the origins of these strange worlds.
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