Our solar system has eight major planets, and nearly 200 moons. Though astronomers have to date found nearly 4,000 planets orbiting other stars, no moons have yet been found. That hasn't been for any lack of looking, it’s just that moons are smaller than planets and therefore harder to detect.

The Hubble and Kepler space telescopes found evidence for what could be a giant moon accompanying a gas-giant planet that orbits the star Kepler-1625, located 8,000 light-years away in the constellation Cygnus. The moon may be as big as Neptune and it orbits a planet several times more massive than... Read more

There may be no shortage of balloon-filled birthday parties or people with silly high-pitched voices on the planet WASP-107b. That's because NASA's Hubble Space Telescope was used to detect helium in the atmosphere for the first time ever on a world outside of our solar system. The discovery demonstrates the ability to use infrared spectra to study exoplanet atmospheres.

Though as far back as 2000 helium was predicted to be one of the most readily-detectable gases on giant exoplanets, until now helium had not been found — despite searches for it. Helium was first discovered on the Sun, and is the... Read more

Using Hubble and Spitzer space telescopes, scientists studied the “hot Saturn” called WASP-39b — a hot, bloated, Saturn-mass exoplanet located about 700 light-years from Earth. By dissecting starlight filtering through the planet’s atmosphere into its component colors, the team found clear evidence for a large amount of water vapor. In fact, WASP-39b has three times as much water as Saturn does. Although the researchers predicted they’d see water, they were surprised by how much they found. This suggests that the planet formed farther out from the star, where it was bombarded by a lot of icy... Read more

Only 40 light-years away — a stone’s throw on the scale of our galaxy — several Earth-sized planets orbit the red dwarf star TRAPPIST-1. Four of the planets lie in the star’s habitable zone, a region at a distance from the star where liquid water, the key to life as we know it, could exist on the planets’ surfaces.

Astronomers using NASA's Hubble Space Telescope have conducted the first spectroscopic survey of these worlds. Hubble reveals that at least three of the exoplanets do not seem to contain puffy, hydrogen-rich atmospheres similar to gaseous planets such as Neptune. This means the... Read more

Using NASA's Hubble Space Telescope to peer deep into the vast stellar nursery called the Orion Nebula, astronomers searched for small, faint bodies. What they found was the largest population yet of brown dwarfs — objects that are more massive than planets but do not shine like stars. Researchers identified 17 brown dwarf companions to red dwarf stars, one brown dwarf pair, and one brown dwarf with a planetary companion. They also found three giant planets, including a binary system where two planets orbit each other in the absence of a parent star. This survey could only be done with Hubble’s... Read more

Travelers to the nightside of exoplanet Kepler-13Ab should pack an umbrella because they will be pelted with precipitation. But it's not the kind of watery precipitation that falls on Earth. On this alien world, the precipitation is in the form of sunscreen.

Ironically, the sunscreen (titanium dioxide) is not needed on this side of the planet because it never receives any sunlight. But bottling up some sunlight protection is a good idea if travelers plan on visiting the sizzling hot, permanent dayside, which always faces its star. Visitors won't find any desperately needed sunscreen on this part... Read more

Don't go looking for the proverbial black cat eating licorice in a coal bin on the planet WASP-12b. Twice the size of any planet found in our solar system, the world is as black as fresh asphalt. Unlike other planets in its class, WASP-12b has the unique capability to trap at least 94 percent of the visible starlight falling into its atmosphere.

The temperature of the atmosphere is a seething 4,600 degrees Fahrenheit, which prevents the formation of reflective clouds on the day side. The planet orbits so close to its host that it is tidally locked, which means that it keeps the same side always... Read more

Only when we fly in a commercial jet at an altitude of about 33,000 feet do we enter Earth's stratosphere, a cloudless layer of our atmosphere that blocks ultraviolet light. Astronomers were fascinated to find evidence for a stratosphere on a planet orbiting another star. As on Earth, the planet's stratosphere is a layer where temperatures increase with higher altitudes, rather than decrease. However, the planet (WASP-121b) is anything but Earth-like. The Jupiter-sized planet is so close to its parent star that the top of the atmosphere is heated to a blazing 4,600 degrees Fahrenheit (2,500 degrees... Read more

Solar flares and associated eruptions can trigger auroras on Earth or, more ominously, damage satellites and power grids. Could flares on cool, red dwarf stars cause even more havoc to orbiting planets, even rendering them uninhabitable? To help answer that question, astronomers sought to find out how many flares such stars typically unleash.

A new study of archival ultraviolet observations from the Galaxy Evolution Explorer (GALEX) spacecraft detected dozens of flares from red dwarf stars. Some flares were weaker than any previously detected. Since smaller flares tend to occur more frequently,... Read more

Astronomers once thought that the family of planets that orbit our sun were typical of what would eventually be found around other stars: a grouping of small rocky planets like Earth huddled close to their parent star, and an outer family of monstrous gaseous planets like Jupiter and Saturn.

But ever since the discovery of the first planet around another star (or exoplanet) the universe looks a bit more complicated — if not downright capricious. There is an entire class of exoplanets called "hot Jupiters." They formed like Jupiter did, in the frigid outer reaches of their planetary system, but... Read more