Alien Solar System Amazes With Seven Earth-Sized Planets

This artist’s impression displays the star TRAPPIST-1 and its planets reflected in a surface. The potential for water on each of the worlds is represented by the frost, water pools, and steam surrounding the scene. The star name comes from The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at La Silla Observatory in Chile, the telescope used to discover the first three of the star’s planets. Credit: NASA/R. Hurt/T. Pyle

It took decades to find the first Earth-sized planet. Now, astronomers have found seven of them in one fell swoop! They orbit the cool (literally cool) red dwarf star, TRAPPIST-1, 40 light years away in the constellation Aquarius. Using ground and space telescopes, the planets were all detected as they passed in front of their parent star, causing its light dip temporarily.

According to a paper appearing earlier this week in the journal Nature, three of the planets lie in the habitable zone and might host lakes or even oceans on their surfaces, giving them the potential to support life. The system has both the largest number of Earth-sized planets yet found and the largest number of worlds that could support surface water.

This picture shows the sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint, 19th magnitude star, a class M star or red dwarf, has only 11% of the diameter of the sun and is much redder in color.  Red dwarfs are extremely common and live long lives, frugally using their nuclear fuel. Planets around red dwarfs could be potentially very ancient. Credit: ESO

Although it would be tempting to label the new worlds Doc, Grumpy, Happy, Sleepy, Bashful, Sneezy, and Dopey after the Seven Dwarfs in Snow White, that’s my wishful thinking. For now, they’re labelled, like all new exoplanets, after their parent star, thus: TRAPPIST-1b, c, d, e, f, g and h in order of increasing distance. Astronomers found that at least the inner six planets are comparable in both size and temperature to the Earth. The seventh and most distant, also similar in size to Earth, appears too cold for liquid water but may very well sparkle with ice.

A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres. Information about the size and orbital periods (how long it takes to orbit TRAPPIST-1) of all the planets is also provided for comparison alongside with our own solar system’s inner planets. The planets’ radii (one-half the diameter) and masses are given in relation to Earth’s. Credit: NASA/R. Hurt/T. Pyle

“This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth!” wrote lead author Michaël Gillon of the STAR Institute at the University of Liège in Belgium. Of the 3,583 alien planets known, this system is one of the most exciting and unique. TRAPPIST 1e, f and g all orbit within the star’s habitable zone, a record finding.

This diagram compares the sun and the inner solar system with the TRAPPIST-1 planetary system. The TRAPPIST-1 dwarf star is much fainter and smaller than the Sun and the planets orbit much closer to their parent star than Mercury does around our sun. Credit: ESO/O. Furtak

The host star is interesting in its own right. With just 8% the mass of the Sun, TRAPPIST-1 is a tiny thing, only a bit bigger than the planet Jupiter, and though relatively near to Earth, it’s extremely faint. That’s good news for its planets, all of which orbit very close to the central fire. Indeed, the entire system would easily squeeze within the orbit of our solar system’s closet planet, Mercury. TRAPPIST-1’s small size and low temperature mean that the energy input to its planets is similar to that received by the inner planets in our solar system.

This is an artist’s impression of the TRAPPIST-1 system, showcasing all seven planets in various phases. When a planet transits across the disk of the red dwarf host star, as two of the planets here are shown to do, it creates a dip in the star’s light that can be detected from Earth. Credit: NASA

Given such rich targets, the next step will be to search for signs of life.  Already the Hubble Space Telescope is being trained on each of these orbs to search for atmospheres. Might we find some of life’s well-known byproducts: methane, oxygen, carbon dioxide? Stay tuned!