Forty light-years away, seven Earth-size planets closely orbit a faint star called TRAPPIST-1. We now know more about this system than any other beyond our own.
1) First we thought there were three
The star we today call TRAPPIST-1 was first discovered in 1999 by astronomer John Gizis and colleagues. At that time, the ultra-cool dwarf star got the unwieldy name 2MASS J23062928-0502285, because it was spotted with the Two Micron All-Sky Survey (2MASS).
Then, in May 2016, scientists announced they had found three planet candidates around this star using the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) in Chile. Only two of the planets were confirmed, however. In honor of this telescope, scientists began referring to the star as TRAPPIST-1.
2) ...then we looked at the atmospheres of two
NASA's Hubble Space Telescope was used to find that TRAPPIST-1b and c were unlikely to have hydrogen-dominated atmospheres like those we see in gas giants. This strengthens the case that these planets could be rocky and possibly hold onto water. This result was published in July 2016.
3) ...but actually there were seven.
Astronomers using NASA's Spitzer Space Telescope and ground-based telescopes discovered that the system has seven planets. Three of these planets are in the theoretical "habitable zone," the area around a star where rocky planets are most likely to hold liquid water. This landmark finding was announced on Feb. 22, 2017.
4) The farthest one could be icy
Researchers determined that the farthest planet from the star, TRAPPIST-1h, orbits its star every 19 days, using NASA's Kepler space telescope. This is still much shorter than the orbit of Mercury, which goes around the Sun every 88 days. But because TRAPPIST-1 is so faint – it outputs only .05 percent the amount of energy of the Sun – planet h receives a lot less heat than Mercury, and may be covered in ice.
5) They are likely older than our solar system
The age of a star is important for understanding whether planets around it could host life. Scientists wrote in an August 2017 study that TRAPPIST-1 is between 5.4 and 9.8 billion years old. This is up to twice as old as our own solar system, which formed some 4.5 billion years ago.
6) They are mostly made of rock
Throughout 2017, scientists worked on creating sophisticated computer models to simulate the planets based on available information. They used additional data from Spitzer, Kepler and ground-based telescopes to come up with the best-yet estimates for the planets' densities. The results are consistent with all of the TRAPPIST-1 planets being mostly made of rock. This result was published in February 2018.
7) …and that is reinforced by looking for atmospheres.
Continued observations with Hubble showed that TRAPPIST-1 d, e and f are unlikely to have puffy, hydrogen-dominated atmospheres, as of February 2018. Scientists will need more data to determine how much hydrogen TRAPPIST-1g has.
8) The planets may appear like moons in each other's skies
The TRAPPIST-1 planets are so close together that if you could stand on the surface of one, you might see some of the neighboring planets hovering above. This idea was inspirational for the TRAPPIST-1 travel poster.
9) We imagine them based on real data
While we can't take photos of the planets themselves, visualization specialists at Caltech/IPAC in Pasadena, California, work with scientists and their data to come up with illustrations of the TRAPPIST-1 system.