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Starless planets: alone in interstellar space

Starless planets: alone in interstellar space

HIt’s an amazing idea: trillions of planets could wander through our Milky Way Galaxy alone, going their own way without orbiting a star. Scientists led by Takahiro Sumi and Naoki Koshimoto of Osaka University in Japan evaluated observational data from Mount John Observatory in New Zealand to learn more about this strange group.

The fact that planets can become homeless is initially apparent due to various interactions in our galaxy: they can be thrown out of their orbit around their parent star by collisions, close encounters with a second, more massive planet, or when a star passes by. Such deadly encounters are said to occur frequently in young planetary systems – and this was also the case in the solar system: Earth’s moon likely formed when the young Earth collided with a celestial body the size of Mars. Uranus’ tilted rotation axis may also be the result of a collision between the planets.

There are many more planets than stars

But astronomers have been debating how many exoplanets exist in our Milky Way Galaxy for years. In 2011, Sumi and his colleagues suspected that for every star in the Milky Way there could be two Jupiter-mass planets. A group led by Louis Strigari of the Kavli Institute at Stanford University in 2012 came to the conclusion that homeless planets outnumber stars by a factor of 100,000. Strigari and his colleagues took into account in particular planets with smaller masses, such as those about the size of Earth. In contrast, work conducted by a group led by Przemek Mroz at the University of Warsaw in 2017 found that large, Jupiter-like orphan planets are eight times rarer than Sumi and his team thought.

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The new analysis, presented in two papers accepted for publication in the scientific journal The Astronomical Journal, now estimates something in the middle: according to it, there are about 20 orphan planets for every star in the Milky Way. That’s much lower than the number estimated by Strigari’s team, but it’s still a significant amount: There are between 100 billion and a trillion stars in the Milky Way — so the number of starless planets in our galaxy is likely to number several trillion.