There's been so much exoplanet news this week! I was in Texas the past couple of days giving a bunch of talks, so I'm trying to catch up. All the exoplanet news is way cool, but too much for one post, so I've split them up. I'll post the other parts shortly. Part 1: A trio of hot little rocks First up? The three smallest exoplanets found so far. I usually don't like to write about incremental discoveries, but this one is truly cool: all three orbit the same star, and all three are smaller than Earth! Any one of these would be a record breaker, but to find all three at once, in the same place? Amazing.

They orbit the star KOI-961 (short for Kepler Object of Interest), and were observed by the Kepler Observatory (details on how that all works can be found here). They all orbit the star extremely close in: the farthest one is a mere 2.3 million km (1.5 million miles) from the star! They're so close they all take less than two days to circle it once. And even though the star is a red dwarf, and therefore relatively cool, they are so close to it that they probably resemble airless, heat-blasted Mercury more than Earth. They are almost certainly rocky/metallic bodies, since they are so small: 0.78, 0.73 and 0.57 times the diameter of the Earth. Although we've been surprised before, it's hard to imagine anything that small could hold onto much atmosphere when they are so hot. Funny, too: the star is tiny, only a bit bigger than Jupiter. And the planets are so close in the KOI-961 system looks more like Jupiter and its moons than our own solar system! The artwork above drives that point home. Everything there is to scale: the relative size of the star, the planets, Jupiter, and its moons.

[Edited to add: Note that the distances are not to scale!]

Why is this news important? Well first, it adds more weight to the idea that planets smaller than Earth exist and can be found around other stars. Second, it shows that red dwarf stars can form and hold onto planets... which itself is important because red dwarfs are by far the most common kind of star in the Universe. They make up roughly 80% of the total number of stars! So finding multiple planets around one means, once again, planets are almost certainly common in the galaxy. And third, it just shows once again that the Universe is a surprising place. This mini-solar system proves that nature is diverse, and will fill any niche it can. It also implies, very strongly, that we need to broaden our concepts of how solar systems form, what they look like, and how they behave. Image credit: NASA/JPL-Caltech

Related posts: - Kepler finds a mini solar system! - Another Kepler milestone: Astronomers find two Earth-sized planets orbiting the same star! - No, it’s *not* the smallest exoplanet found! - A boiling superEarth joins the exoplanet roster