Planet: Kepler-1655 b
Discovered by: Raphaëlle D. Haywood et al.
Date: April 6, 2018
Key Facts: This new planet, discovered a whopping 750 light-years away, is twice the size of Earth. That usually would make it a "super Earth," but its discoverers believe that it may be a Neptune-like world, shrouded in gas. It orbits a star like the Sun, with a short year that lasts only 12 days.
What's new: This planet is a cool clue that helps answer the question: how big do planets have to be before they're gas giants? Not that big, apparently. Kepler-1655 b (the new planet) is only two times the size of Earth, but it's probably a gas planet like Neptune. That means that planets have to be pretty close to the size of Earth to be rocky worlds– recent studies think planets need to be 1.6 times the size of Earth or smaller to be terrestrial worlds. Our own solar system doesn't have a planet that's between the size of Neptune and Earth– but planets that size are actually the most common planets in the galaxy. These worlds are called "super Earths" or "mini Neptunes." And we're still figuring out if those planets are rocky or gassy. It's a space mystery, waiting to be solved.
See details and check the official NASA planet count at the NASA Exoplanet Archive