The planets orbit very close to their host star. All seven of them can actually fit within the orbit of Mercury.

news Science Saturday, February 25, 2017 - 10:21

Scientists have discovered a star system 39 light years away that has seven earth-sized planets orbiting it. Three of these planets are in the habitable zone and are capable of holding liquid water. The announcement of this discovery was met with great excitement. The team responsible for the discovery was headed by University of Cambridge astronomer Didier Queloz, who incidentally also discovered the very first planet outside our solar system.

Planets orbiting other stars are called extra-solar planets or exoplanets. Most such planets we’ve found are large gas giants the size of Neptune orbiting very close to their host star. This exoplanet system is called TRAPPIST-1, named for The Transiting Planets and Planetesimals Small Telescope in Chile. In any such system, the star is given the suffix ‘A’, and the planets are given alphabets from b. Therefore, the star here is called TRAPPIST-1A (the A is usually dropped), and the seven planets number from TRAPPIST-1b to 1h, in order of discovery and not distance from the star.

(Artist’s conception showing what the TRAPPIST-1 planetary system may look like, based on the data we have about the planets’ mass, size, and distance from the star. Photo by NASA/JPL-Caltech)

The planets orbit very close to their host star. All seven of them can actually fit within the orbit of Mercury. Their orbital period ranges from 1.5 days to 20 days. They are so close to each other that they constantly gravitationally tug each other. The sizes of the planets are similar, and in fact, very close to the diameter of earth. Additionally, even their masses are close to earth’s and this leads to the natural conclusion that nearly all of them are rocky, prompting the adjective “earth-like”.

Three planets, TRAPPIST-1e to 1g, exist bang in the middle of the system’s habitable zone. The star is an ultra cool, M-type dwarf star. These kind of stars are not very bright and emit energy mostly in the infrared. Therefore, their habitable zone lies very close. However, being so close to the star means that the planets are tidally locked: much like our moon, only one side of the planet faces the star, while the other is in perpetual darkness. Such a scenario would imply that the tidally locked planets are hostile to life. The star-facing side would be burned and barren, while the darker side would be extremely cold. But the presence of an atmosphere could fix that by redistributing heat.

“It’s the first time so many Earth-type planets [have been found] formed in the habitable zone of a star,” Michaël Gillon, an astrophysicist at Université de Liège in Belgium and study co-author, said during a NASA press briefing Tuesday.

(Top row: artist conceptions of the seven planets of TRAPPIST-1 with their orbital periods, distances from their star, radii, and masses as compared to those of Earth. Bottom row: same data about Mercury, Venus, Earth, Mars. Photo by NASA/JPL-Caltech)

Being in the habitable zone and being earth-sized does not necessarily mean containing life. Life took a long while to kickstart on our planet. The earth formed 4.5 billion years ago and the oldest evidence of life we have is from 3.8 billion years ago, and that too without oxygen. The first multicellular organism appeared, just 1 billion years ago. TRAPPIST-1 is a young star. It is just 500 million years old. Any life there would actually not be life as we know it; in the worst case, the planets are scorched and barren, in the best, there are present in pockets the precursors to life, RNA molecules.

Before scientists think of exploring these planets, that are close to us on a cosmic scale but are still a whopping 39 light years away, the next practical step would be to improve our technology to find planets close enough that we can study their atmosphere from here. Traditionally, scientists have always looked for stars similar to our sun to find exoplanets capable of harboring life. A recent group of discoveries show that smaller, cooler stars, such as TRAPPIST-1 and our closest neighbor Proxima Centauri, are also capable of holding habitable planets.

The system’s existence further suggests that star systems with earth-sized planets are much more common than we thought. Most previous exoplanets we’ve discovered have been gigantic gas balls, the size of Neptune and even Jupiter. Scientists have always claimed that this is observation bias -- giant planets are easier to detect because of their size. TRAPPIST-1 seems to confirms this.