Astronomers have found a clutch of free-floating rogue planets, just out there in space on their own, in a nearby star-forming region. They tagged from 70–170 of them.
Free-floating planets are advertised as Starless planets, which are doomed to wander into space. We don't know much about them. It's difficult to get a census of the first ones because they tend to be discovered one by one.
The ones found are very young, so recently born that they're still very hot from the formation process. The human eye can't see the wavelength of light that they glow in, but telescopes and cameras can. Most of the ones known are young because they get harder and harder to detect as they age.
They can be identified in many ways. A key point is knowing a planet's age: Because they cool as they grow older, they shine at different wavelength of light depending on their mass, so a bigger planet will be hotter and brighter at a given age. We have models of how bright an object should be, so that can be used to estimate its mass.
A group of people looked toward Scorpius and Ophiuchus, a pair of constellations where a huge star-forming cloud lies. Why there? We don't know how all free-floating planets form, but we're pretty sure they come from gas and dust clouds that collapse on their own, similar to the way stars are born, or they form around stars and get ejected. If a planet gets too close to another one it can be ejected from its planetary system.
This region is one of my favorite areas of the sky, and I will always take the opportunity to show off a photo of it taken by a photographer.
Would I tell the truth to you? Wow. It has a lot of scientific value.
The astronomer looked at the sky for over two decades and found over 26 million objects, about 850 times the area of the full Moon. The Dynamical Analysis of Nearby ClustErs is a project that became their catalog to search in.
They turned to the European Space Agency's Gaia mission, which has mapped the positions, motions, distances, and colors of a billion stars. They used that to find objects in the catalog that could be in the Sco-Oph cloud. About 20% of the objects they found were not previously identified as being part of the star-forming region.
It's difficult to pick out the objects with planetary mass. The objects forming in the Sco-Oph cloud have a range of ages from 1 to 10 million years. They took a number of specific values for age and calculated their mass based on them. At least 70 and as many as 170 of their objects have the mass of a planet, depending on the age assumed.
This is significant. Jupiter has a mass up to 13 times that of a planet. An object around that mass can cause enough pressure in its core to cause the decay of deuterium. The definition of a true star is when brown dwarfs run from 13 to 75 times Jupiter's mass, and they can combine hydrogen and helium.
Brown dwarfs form like stars when they collapse from gas and dust. At the low end, they can form like planets, with material from a disk of debris around a newly forming star. The more we know about these free-floating planets, the better we can understand how they form.
Knowing how many are born from a given cloud is part of this. The Sun is a low-mass red dwarf, which makes up less than 10% of all stars. A much smaller fraction is made up of giant stars. The initial mass function is the basis of our understanding of how stars and galaxies behave.
The first brown dwarf wasn't discovered until the 1990s, so it's not well known at the very low mass end. It's important to have a complete census of a single star-forming region so we know what the IMF is. The purpose of this search is to get a better handle on the IMF.
They found that of the 3,500 objects, 4.5% are rogues, with a somewhat large uncertainty due to the age of the objects not being known well. That is very helpful. It's amazing that we can find such things.
I smile whenever I write about rogue planets. When I was a kid, I would watch science fiction shows on TV and sometimes the plot would be about a rogue planet. I remember thinking that was silly. How could such a thing exist?
Nature is more imaginative than I was. It makes lots of rogue planets, and the fact that they exist tells us something about the history of planetary systems. How many planets did the Sun have when it was young, and how many of them were thrown out into the universe for billions of years?
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