Illustration of a solar system with seven Earth-sized planets.

An illustration of the TRAPPIST-1 system. The seven planets of TRAPPIST-1 are all Earth-sized and terrestrial, according to research published in 2017 in the journal Nature.

Year in review: Top 5 NASA moments of 2017

NASA's Exoplanet Exploration Program

Feature | December 30, 2017

For New Year’s Eve, we’re sharing the top moments of the past year in NASA’s search for life and planets outside our solar system. Here are a few of the most exciting discoveries of 2017, as we count down to a new year, and new worlds.

The TRAPPIST-1 system consists of seven Earth-sized planets orbiting a red dwarf star.
Credit: NASA/JPL-Caltech

1) Most Earth-size planets in the habitable zone

This year brought us one of the most exciting discoveries yet: a system of seven worlds that sets a new record for the greatest number of habitable-zone planets found around a single star. All seven of TRAPPIST-1 planets could have liquid water under the right atmospheric conditions, but the chances are highest with the three in the habitable zone. The three planets in the habitable zone are in the area around the parent star where a rocky planet is most likely to have the temperature for liquid water– which is key to life as we know it. But recent findings suggest life would have an uphill battle on a planet close to a red dwarf star like TRAPPIST-1, largely because such stars are extremely active in their early years—shooting off potentially lethal flares and bursts of radiation.

More: NASA telescope reveals largest batch of Earth-size, habitable-zone planets around single star

2) First alien system with eight planets

Comparison of the Kepler-90 system and our solar system
Credit: NASA/Ames Research Center/Wendy Stenzel

Our solar system now is tied for most number of planets around a single star, with the recent discovery of an eighth planet circling Kepler-90, a Sun-like star 2,545 light-years from Earth. The newly-discovered Kepler-90i – a sizzling hot, rocky planet that orbits its star once every 14.4 days – was found in data from NASA’s Kepler Space Telescope, using machine learning from Google. In this case, computers learned to identify the signals caused by planets by finding instances in Kepler data where the telescope recorded changes in starlight caused by planets beyond our solar system, known as exoplanets.

More: See the Kepler-90 system in 3D

Movie of four exoplanets around their star
Credit: Jason Wang/Christian Marois

3) First movie of four exoplanets

The technology to directly image exoplanets is still in early stages, but the results are already spectacular. This new movie shows four planets more massive than Jupiter orbiting their star HR 8799, more than 100 light-years from Earth. The movie is actually a series of images taken over seven years at the W.M. Keck Observatory in Hawaii. Only a small fraction of the planets' full orbits are shown over this time period. The closest-in planet circles the star in around 40 years; the furthest takes more than 400 years.

More: See "Many Worlds"

This illustration shows the seething hot planet Kepler-13Ab that circles very close to its host star
Credit: NASA/ESA/G. Bacon (STScI)

4) Planet that snows sunscreen

A delightfully strange discovery of 2017 was made by NASA’s Hubble Space Telescope: a blistering hot planet outside our solar system where it "snows" sunscreen. The problem is the sunscreen (titanium oxide) precipitation only happens on the planet's permanent night side. Any possible visitors to the exoplanet, called Kepler-13Ab, would need to bottle up some of that sunscreen, because they won't find it on the sizzling hot, daytime side, which always faces its host star.

More: Hubble observes exoplanet that snows sunscreen

This illustration shows one of the darkest known exoplanets — an alien world as black as fresh asphalt.
Credit: NASA/ESA/G. Bacon (STScI)

5) Darkest world

If you like oddball planets, here’s a contender for the weirdest. In 2017, NASA's Hubble Space Telescope observed a pitch-black exoplanet that’s the shape of an egg. WASP-12b looks as black as fresh asphalt because it eats light rather than reflecting it back into space. The doomed Jupiter-like planet is also being stretched into the shape of an egg by the tidal forces of its extremely close star, which will ultimately destroy it.

More: NASA's Hubble captures blistering pitch-black planet

Close-up of spectrum of van Maanen's Star.
Credit: Carnegie Institution for Science

Honorable mention: 100 years since the first evidence of exoplanets

This year marked the 100th anniversary since the first evidence of planets outside our solar system. But the photographic proof of this astonishing discovery was overlooked and buried in the Mount Wilson Observatory archives, because no one knew exoplanets existed. The data was only rediscovered and published last year.

More: See the first exoplanet evidence