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.
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.
Search using gravitational lensing
Sumi, Koshimoto and their colleagues rely on the most comprehensive observational data to date. The 1.8-meter telescope on Mount John recorded more than 6,000 so-called “microlensing events” between 2006 and 2014. This refers to the brief flash of light from a distant star when a celestial body, such as an orphan planet, passes directly in front of it from Earth’s perspective. According to Einstein, the “miniature gravitational lens” bends space in such a way that the light from the distant star is measurably amplified over the course of hours, days or weeks, depending on the movement of the object. By observing the amplified light, the mass of the invisible celestial body passing between the star and the observer can be calculated.
According to the researchers, out of 6,000 events, about 3,500 were triggered by orphan planets. Only one of them was the size of Earth. But in reality, Earth-like planets should be the vast majority of orphan planets, the authors write: The smaller the planet, the greater the chance it has of being pushed out of its orbit by a larger planet and ending up becoming an orphan. However, Earth-sized planets produce only short-term increases in starlight and are therefore often overlooked.
Jupiter-sized objects are easy to spot, but are actually rarer when isolated than Earth-sized objects. If NASA launches its Nancy Grace Roman space telescope in 2027 as planned, the number of known Earth-sized orphan planets could double. The planned infrared telescope will use the microlensing method to specifically look for planets in the Milky Way, especially those orbiting their stars, but also those without a star. If the analysis by researchers led by Sumi and Koshimoto is correct, the telescope should find about 400 orphan planets roughly the size of Earth.
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