Stargazing in April: A planetary hit-and-run

There are a number of space oddities in our solar system, writes Nigel Henbest. And they may all be related

Mercury: hit-and-run culprit of the Solar System? (NASA)

This month, we have not just one “evening star”, but two. Both the planets that lie closer to the sun are on view after sunset. Venus is so brilliant that it outshines everything in the sky, bar the sun and the moon. Down in the dusk twilight you’ll find fainter Mercury, which never strays far from the sun.

Space probe missions to these worlds have shown them both to be space oddities. Mercury is made almost entirely of iron, like a planet-sized cannonball with only a thin veneer of overlying rock. Venus rotates very slowly, and the opposite way to the other planets. While we’re on the subject, our own planet is pretty unusual too: it has a companion so large that astronomers view the Earth-moon system as a double-planet.

Now researchers are beginning to think that all these odd characteristics could be related. It involves several cosmic hit-and-run incidents when the planets were young and reckless, with Mercury as one of the prime culprits.