Homeless wanderers: Astronomers have found between 70 and 170 lonely planets in a close cluster of young stars – more than ever before. These planets do not orbit mother stars, but move in isolation through space. The large number of these loners in a relatively small space sheds new light on the possible mechanisms of formation of such starless exoplanets, as the researchers report in the journal “Nature Astronomy”.
Most of the planets known to date are part of a system: They orbit one or more stars and were formed in the rotating gas and dust disk around their star. But there is also loner – unbound planets that drift through space without a mother star. Among these starless planets are both earth-sized rock planetsn as well big gas giants, some even bordering on brown dwarfs.
Targeted manhunt
Astronomers working with Nuria Miret-Roig from the University of Bordeaux have now tracked down the largest population of starless exoplanets to date. For their study, they evaluated observational data from 20 years and from numerous telescopes in order to search for hidden solitary planets in the nearby star association Upper Scorpius. It is around 420 light years away and contains numerous first young stars a few million years old.
Because starless planets do not reflect light, the research team searched the more than 80,000 images for the weak heat signatures of still young loner planets, but also for telltale movements of surrounding stars. “We measured the tiny movements, the colors and the brightness of tens of millions of sources in a large area of the sky,” explains Miret-Roig.
Up to 170 starless hikers
The yield was surprisingly large: the team was able to track down between 70 and 170 lonely planets in the two regions. The wide range comes from the fact that there is great uncertainty about the age of the heavenly bodies. But because age determines their heat radiation and thus brightness, some of the lighter objects could possibly already be brown dwarfs.
But even assuming only 70 starless exoplanets, this is the largest collection of such loners ever identified. “We didn’t know how many to expect and are thrilled to have found so many,” says Miret-Roig. Most of these exoplanets are gas giants at least the size of Jupiter. The astronomers were also unable to detect any particular abnormalities in the distribution – the planets were relatively evenly distributed over the observation field.
A starless planet in every square degree – at least
The number of solitary planets allows conclusions to be drawn about how often such starless wanderers occur. In the examined section of the sky, the proportion of such exoplanets compared to stars and brown dwarfs was 0.045, as the astronomers report. On average, there were 0.5 to one starless planet per square degree in their field of observation.
From this it can be deduced that there are probably far more such lonely planets in space than long assumed. “There could be billions of these giant free-floating planets that roam the Milky Way without a host star,” explains Roig’s colleague Hervé Bouy. That fits in with earlier estimates that it up to 50 billion such planets could exist in our galaxy.
On the trail of the loner planets in space.© ESO
How did the loner planets come about?
The large number of these heavenly bodies raises the question of how such loners arise. Some astronomers believe that most of them were once formed in planetary systems. Later, gravitational turbulence caused by co-planets or nearby stars ensured that they were thrown out of their system. Other researchers believe it is more likely that the loners were created by the local collapse of gas clouds – similar to the way stars are formed.
“Our large sample of isolated exoplanets offers us an excellent opportunity to test the theories of star and planet formation,” the researchers explain. As they found, current models of gas cloud collapse show a much lower proportion of loners than observed. “You predict a proportion of 0.009 to 0.019 – that is seven times less than our measurements show,” says Miret-Roig.
The astronomers estimate that at least ten percent of their starless planets, and probably more than 30 percent, did not originally emerge as loners. Instead, they were probably part of a planetary system and were subsequently ejected. “Planets with the mass of Jupiter are relatively difficult to throw out, which means that there could be even more smaller loners of the mass of the earth in our galaxy,” says Miret-Roig. (Nature Astronomy, 2021; doi: 10.1038 / s41550-021-01513-x)
Source: European Southern Observatory (ESO)