A Guide to the Solar System’s Biggest Secrets

Astronomers have spent centuries filling in their sketches of our corner of the Milky Way. But these maps, like all maps, are only approximations of reality. Their blind spots are likely home to unknown entities – bodies too small, too close to the sun, or too far away for us to see. Here are some celestial objects that astronomers have suspected of having dodged their telescopes over the years.

Vulcan and the vulcanoids

Astronomers once interpreted an oddity in Mercury’s orbit as a sign of a planet hidden in the sun’s rays. Einstein’s theory of gravity explained it, but the area could harbor asteroids (“vulcanoids”). They would be small: the two STEREO spacecraft for observing the sun of NASA would have been spotted beyond a few kilometers.

Lost Ice Giant

In digital reconstructions of its early days, the solar system becomes rowdy. Near-collisions between planets end with Jupiter sending Uranus or Neptune in 99 out of 100 simulations. Yet both remain. An explanation: a third body took the hit. Calculations suggest that a massive, icy planet could have fought with Jupiter and lost.

Planet 9

Hundreds of dots appear clustered beyond Neptune, a hint that something up to 10 times more massive than Earth could lie beyond. A large planet could provide the gravitational influence needed to attract them. New evidence of this celestial body – possibly a rogue world of interstellar space – was unveiled in 2016.


Semi-regular extinctions on Earth suggest that a faint companion star may have periodically passed, flooding us with meteorites swept along in its wake. But such a brother is gone. A study of the sky in 2010 found thousands of new stars, just six light years away. Nemesis, who would be a quarter of that distance, was absent.

This story appears in the Fall 2020, Mysteries issue of Popular science.

Arline J. Mercier

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