Neptune’s own existence was a puzzle until recently. The dusty cloud that gave birth to the planets probably thinned out further from the sun. With building material so scarce, it is hard to understand how Uranus and Neptune, the two outermost planets, managed to get so big.
Uranus and Neptune formed much closer to the sun and migrated outwards, possibly swapping places in the process.
That would have left behind enough material just beyond their birthplace to form a planet with twice the Earth’s mass, according to calculations published in 2008 by Steven Desch of Arizona State University in Tempe.
Neptune’s peculiar moon Triton may once have been paired with this hypothetical super-Earth, Desch and colleague Simon Porter now say. Triton is larger than Pluto, and it moves through its orbit in the opposite direction to Neptune’s rotation, suggesting that it did not form there but was captured instead.
For Neptune to capture Triton, the moon would have had to slow down drastically. One way to do this is for Triton to have had a partner that carried away most of the pair’s kinetic energy after an encounter with Neptune. In 2006 researchers argued that Triton was initially paired with another object of similar size that wound up being gravitationally slung into space after the pair ventured near Neptune
But Douglas Hamilton of the University of Maryland, College Park, one of the authors of the 2006 study proposing that Triton had a long-lost twin, points out that smaller bodies would have been common in the early solar system, before planet migration cleared many of them away.
Neptune would therefore have had many opportunities to snag Triton from one of these punier objects, rather than from a much rarer super-Earth, so that explanation may still be more likely, he says. Even so, he is not ready to rule out Desch’s idea: “It’s worth pursuing to see where it will lead.”