When the sun was young, it may have stolen a planet from another nearby star – meaning an exoplanet could be waiting on the outer edges of the solar system for us to discover
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| The sun may have stolen a planet from near by star (Credit: European Southern Observatory) |
A Mars-sized planet stolen from another star system may be hiding on the edges of our own, according to a model of planetary evolution. While the existence of such a planet remains to be verified, if it does exist, it would offer a way to visit an exoplanet without having to travel the vast distances between the stars.
“You need everything to line up incredibly well to be able to find one of these things because they’re going to be so faint, but the implications are enormous,” says Amir Siraj at Princeton University. “Imagine sending a spacecraft to an exoplanet in the solar system and all that we can learn from that.”
Siraj’s prediction is built on emerging research into baby star clusters, where most planets are formed. Before heading out on their own, stars are usually born in tight-knit broods bound by gravity, allowing them to interact with each other and any nascent planets. Such close encounters between stars can catapult worlds from their orbits, unleashing free-floating planets that may ultimately get caught up in the gravitational coat-tails of another star.
A recent survey of these free-floating exoplanets confirmed that small rocky worlds like Mercury or Mars are more likely to be tossed from their home systems compared with more massive Earth-sized worlds or gas giants. By merging these two lines of research, Siraj estimated how many small planets the sun might have picked up while it was still in its stellar nursery billions of years ago, before it set out on its own.
The results suggest that the outer solar system probably contains one to three planets comparable in mass to Mars and about two to five planets comparable in mass to Mercury. These predicted worlds could be hundreds or thousands of astronomical units (AU) away, where 1 AU is the distance from Earth to the sun. But Siraj says that the Vera C. Rubin Observatory, which is slated to become fully operational in 2025, might be able to spot any in the range of 400 to 700 AU.
Claims of undiscovered planets in the solar system aren’t new, but Siraj says his prediction is distinct from speculation about Planet Nine, a much larger hypothetical world that some astronomers think might explain odd orbital anomalies beyond Neptune. If both ideas are true, the outer solar system could be hosting a surprisingly large number of planets.
Richard Parker at the University of Sheffield, UK, says that Siraj’s conclusions appear sound, though he noted that more research is needed to constrain the probability of finding any adopted worlds. If the planet does exist, the opportunity to study an exoplanet up close could shed light on a host of tantalising questions, including the mystery of how life emerged on Earth and whether it exists elsewhere, says Parker. “It would help us understand a lot more about how planets form in our own solar system, and how we came to be.”
Journal reference: The Astrophysical Journal Letters, in press