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Lots of starless exoplanets – astronomers discover up to 170 lonely small planets at once

Hobo wanderers: Astronomers have discovered between 70 and 170 lonely planets in a group close to young stars — more than ever before. These planets do not revolve around their parent stars, but rather move in isolation through space. The large number of these solitary in a relatively small space sheds new light on the possible mechanisms of formation of such starless exoplanets, 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 are formed in the disk of gas and dust rotating around their star. But there is also lonely Unbound planets drifting through space without a parent star. Among these starless planets are both Earth-sized rocky planetsn also Big gas giants, some even overlook brown dwarfs.

The red circles mark the locations of 115 newly discovered starless planets. © ESO / N. Risinger (

Targeted pursuit

Astronomers working with Purdue University’s Nuria Merritt Roig have tracked the largest number of starless exoplanets to date. For their study, they evaluated observational data from 20 years and from several telescopes in order to search for solitary planets hidden in the nearby upper star association Scorpio. It is about 420 light-years away and contains many young, first stars that are a few million years old.

Because starless planets do not reflect light, the research team searched more than 80,000 images for the weak thermal signals of isolated planets, but also for the motions of surrounding stars. “We measured the precise motions, colors, and brightness of tens of millions of sources in a large expanse of sky,” explains Miret-Roig.

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Up to 170 non-star parks

The payoff was surprisingly large: the team was able to track between 70 and 170 isolated planets in the two regions. The wide range comes from the fact that there is a great deal of uncertainty about the age of celestial bodies. But since age determines their thermal radiation and therefore their brightness, it is possible that some of the lightest objects are brown dwarfs.

But even assuming there are only 70 exoplanets, this is the largest group of such isolated planets ever identified. “We didn’t know how many people to expect and we’re glad we found so many,” Merritt Roig says. Most of these exoplanets are gas giants at least the size of Jupiter. The astronomers were also unable to detect any particular anomaly in the distribution – the planets were distributed relatively evenly over the observation field.

A planet without stars 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 ratio of these exoplanets compared to stars and brown dwarfs was 0.045, according to the astronomers’ report. On average, there were 0.5 to one starless planet per square degree in the observation field.

From this it can be concluded that there are likely to be many more such isolated planets in space than has long been assumed. “There may be billions of these free-floating giant planets roaming the Milky Way without a host star,” explains Roig’s colleague, Hervé Bowie. This is consistent with previous estimates Up to 50 billion dollars Such planets can exist in our galaxy.

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On the trail of lonely planets in space.© ESO

How did the lonely planets arise?

The large number of these celestial bodies raises the question of how these solitary ones appeared. Some astronomers believe that most of them formed in planetary systems. Later, gravitational perturbation caused by common planets or nearby stars expelled them from their system. Other researchers believe that the lone objects likely originated from the local collapse of gas clouds – similar to the way stars form.

“The large sample of isolated exoplanets provides us with an excellent opportunity to test theories about the formation of stars and planets,” the researchers explain. As they found, current models of gas cloud collapse show a much smaller proportion of recluses than observed. “You’d expect a ratio of 0.009 to 0.019 – seven times lower than what our measurements show,” says Merritt Roig.

Astronomers estimate that at least ten percent of their star-free planets, and perhaps more than 30 percent, did not originally appear as isolated units. Alternatively, they may have been part of a planetary system and were subsequently expelled. “Jupiter-mass planets are relatively difficult to get rid of, which means there may be more small, Earth-mass secluded planets in our galaxy,” Merit Roeg says. (Natural Astronomy, 2021; doi: 10.1038/s41550-021-01513-x)

Cowell: European Southern Observatory (ESO)