A planet that orbits a star outside the solar system is an exoplanet.
Imagine a place with not one, not two, but 7 Earth-sized planets orbiting a single star. TRAPPIST-1 is an Ultra-Cool Dwarf Star. This artist's concept shows what the planet might look like. To learn more about the how artists' took tiny bits of data and made a vivid picture, read here.
TRAPPIST-1 is a system about 40 light years away from Earth (12 parsecs) in the Constellation, Aquarius. The 7 Earth-sized Exoplanets are currently (March, 2017) labeled, "b, c, d, e, f, g, and h". In order based on proximity to their star ("b" is closet to TRAPPIST-1). The Largest planets, g and b are about 10% larger than the Earth. The Smallest planets, d and h are about 25% smaller than the Earth. According to NASA/Caltech's Exoplanet Archive, as of 2017 there are more than 3,450 confirmed exoplanets in the Milky Way Galaxy.
It is likely that most, if not all, of the exoplanets keep the same one side of their surface facing their star at all times. This phenomenon is called Tidal Locking. This is the same phenomenon we observe with our on Moon in Relation to the Earth. Only one side is ever facing us. On the exoplanets this would cause massive temperature differences across their surfaces. This also could, (given the right circumstances) mean there is a possibility of finding liquid water on any of these exoplanets. This is why each of these exoplanets on SOS appears to have one side in shadow. Unlike our moon, the entirety of these planets would likely not ever see the sun.
Though we do not believe any of these exoplanets have moons, as they are too close to their star, if you were to stand on the surface of one of these planets, you would clearly see the other planets in the system and some of them, at certain times would appear even larger than our moon appears to us. This is because TRAPPIST-1 is only a little larger than Jupiter, and its planets orbit only a little further away than that of Jupiter's moons.
Interplanetary Trips would be measured in Days in TRAPPIST-1, unlike in our solar system, where we have to measure them in Months and Years. It only takes 1.5 days for the innermost planet to orbit it's star.
In May 2016 astronomers using the Trappist Telescope (x2 Belgian optic robotic telescope) in the La Silla Observatory in Chile first identified 3 Earth-Sized planets. The Spitzer Space Telescope then followed-up on this discovery. Spitzer was uniquely up to the challenge as it is finely sensitive to the cool glow of the dwarf star. Spitzer confirmed 2 of the three planets and found the 3rd planet to actually be 3 different planets, it then went on to discover 2 more planets, leading to a total of 7 exoplanets. NASA published their findings in the journal, Nature on Feb. 23, 2017.
Spitzer studied the TRAPPIST-1 Star for over 21 days nearly continuously (500 Hours), pausing only to send data back to Earth. Spitzer looked for tiny dips in the star's brightness, as the circling planets passed in front of the star while in transit. Hubble then followed up on Spitzer's data to peer into the system to look for the chemical fingerprint of Hydrogen gas in the atmospheres of the planets. So far Hubble has not found evidence for hydrogen gas in the atmospheres, which is a good indicator that these planets are not gas planets, but terrestrial, rocky bodied planets.
Spitzer Space Telescope launched in 2003 was designed to last at least 2.5 years. 13 years (2016) later Spitzer has operated far beyond the scope of its original mission. Spitzer uses infrared vision (IR) to peer into the cosmos and see it in new ways. It is able to see through the dust in space. Because of this, it originally was used to peer through the dust and debris, deep into stellar nurseries where stars are born. It has been more recently used to map IR temperature of exoplanets, create a 360 degree panorama image of the Milky Way, find a new ring of Saturn hundreds of times larger than any other ring previously known. Spitzer is now expected to last past the launch of its successor, the James Webb Space Telescope, in 2018.
Already more than 130 million miles away, Spitzer faces communication challenges with NASA because of distance. In mid-2009 the telescope ran out of coolant, however the engineers' design still allows it to operate one of its three cameras. (Coolant is needed so the ship's own heat and IR do not interfere with the data collected by the cameras. For all three cameras to function as designed, the temp had to remain at just 5 degrees above absolute zero.)
Unlike other orbital telescopes like Hubble, Spitzer was design to move away from the Earth slowly, in a Earth-Trailing orbit. (Because of the massive amount of IR light waves that the Earth would radiate onto the telescope, making it hard to see clearly through.)