Smallest Exoplanet

For many years searches for exoplanets were dominated by giant hot Jupiters. Where were the small planets? In recent years this has changed as astronomers’ detection techniques have become more sensitive to smaller planets and the smallest exoplanet discovered so far is Kepler-42d.

This midget world lives around a red dwarf 126 light years away, revealing itself to astronomers through the tiny dip in light it causes in its parent star as the planet moves in front of it. It has a radius just 0.57 times that of Earth, making it about the same size as Mars. Kepler-42d’s mass is just less than Earth’s at 0.9 Earth masses.

Red dwarf stars are cool, miniature stars and their attendant planetary systems are similarly scaled down. Kepler-42d is the outermost planet of three known in orbit around their star, and yet it is still so close to its star that its year lasts just less than two Earth days.

At this proximity Kepler-42d has a surface temperature of at least 181 degrees Celcius, assuming no atmosphere. This is far too hot for life as we know it.

However, discovering rocky planets of any type is important because it helps tell us that planets the size of Earth or smaller are common in the universe. Smaller, rocky planets have the best chance of being habitable, provided they live in their star’s habitable zone and they have an atmosphere and water.