Akihiko Fukui

The three planets in the Kepler-51 system have long intrigued astronomers: they are Saturn-sized but less dense than water. These 'super-puff' worlds, with masses no more than ten Earths, are like cosmic dandelions — massive yet almost weightless. Their orbits are arranged like musicians in an orchestra, each playing its part, beating the rhythm with transits.

Two new planets are circling dim red stars. One is nearly the size of Earth, the other is three and a half times larger. Both lie in what astronomers call the warm zone: a region where starlight doesn't scorch the surface but also doesn't leave it in perpetual cold. The host stars are red dwarfs, the most common stars in the galaxy.

350 light-years away, four planets orbit the star V1298 Tau. They are 5–10 times larger than Earth but lighter than they appear. For nine years, astronomers tracked their motion to understand what they are made of and how they lived in the past. The planets are lined up in a chain: their orbital periods are nearly in 3:2, 2:1, and 3:1 ratios.

A family moves into a cramped apartment in the city center. If the move was calm, relatives might have moved in with them. But if the family was evicted from their old home by a storm, they'll end up alone in the new place.

When a distant planet passes in front of its star, it briefly blocks part of the light. Astronomers call this a transit and judge the planet's size by the depth of the dip. But light has a property: a massive body acts like a lens. Even a giant planet can slightly bend the star's rays, and the dip becomes a bit shallower than you might expect.

A quarter-century ago, Michel Mayor and Didier Queloz discovered the first hot Jupiter, and shattered all the rules of planet formation. A gas giant on a days-long orbit couldn't have formed next to the star: it's far too hot and lacks building material. So, it must have been born far away and then migrated.