Astronomers have confirmed the existence of the seventh planet around the ultracool dwarf star TRAPPIST-1.
The modest M8 red dwarf star TRAPPIST-1 became famous after astronomers discovered seven small exoplanets in orbit around it. At the time the discoverers made the announcement in February, they couldn’t say much about the outermost world, labeled h: The astronomers had seen the planet — or, at least something they thought was a planet — pass in front of the star only once.
Rodrigo Luger (University of Washington, Seattle) and colleagues, including members of the original discovery team, have now confirmed planet h’s existence and some of its specs.
The team used more than 70 days of data from NASA’s repurposed Kepler spacecraft, taken as part of its K2 mission. The craft detected h crossing in front of its star four times, with an orbital period of 18.77 — just what the researchers were expecting, based on their previous observations. (They analyzed the data three different ways, too, just to be sure.) This orbit places the exoplanet well outside TRAPPIST-1’s habitable zone: The amount of energy planet h receives from the little star is on par with what dwarf planet Ceres receives from the Sun at its home in the main asteroid belt.
The transits reveal that planet h is 75% as wide as Earth, or about 40% larger than Mars. But we still don’t know the world’s mass. Researchers used tiny shifts in the other six exoplanets’ transit times to estimate their gravitational influence on one another, and hence their masses. Unfortunately, planet h’s measured transits aren’t clean enough to reveal timing shifts due to its siblings’ gravitational tugs, Luger says.
The exoplanet’s orbital period makes a complicated pattern with the periods of those around it, the authors explain May 22nd in Nature Astronomy. Normally, when we talk about such resonant orbits, we think of situations like that of Jupiter’s Galilean moons: For every circuit Ganymede makes around Jupiter, Europa makes two. TRAPPIST-1’s planets have a more complicated arrangement, called a higher-order Laplace resonance, in which the pattern is a combination of three periods that doesn’t exactly produce the straightforward, integer multiples we usually think of. For those interested in the math, the relationship is
x/P1 – (x+y)/P2 + y/P3 = 0
where x and y are integers and P1, P2, and P3 are the orbital periods of planet 1, planet 2, and planet 3 in the trio of neighboring bodies you’re comparing.
For those not interested in the math, just know that for every two laps planet h makes around TRAPPIST-1, planet g makes about 3, and planet f makes (more roughly) four. The exoplanets would have migrated into this complex chain arrangement sometime after the system formed, then gotten gravitationally stuck.
The above animation shows a simulation of the TRAPPIST-1 exoplanets over 90 Earth-days, then focuses on the outer three after 15 days. The three-body resonance of the outer three planets causes the planets to repeat the same relative positions. Astronomers used this expected resonance to predict the orbital period of TRAPPIST-1h. Credit: Daniel Fabrycky / University of Chicago
How Old Is TRAPPIST-1?
Luger’s team also tried to constrain TRAPPIST-1’s age. Dating stars as puny as this one is tough. The way a star ages depends on its mass; at a measly 8% the Sun’s mass, TRAPPIST-1 will age very slowly.
Thanks to the K2 data, the astronomers could use starspots to clock the dwarf’s rotation period at 3.3 days (about twice as long as the period we previously reported). That’s middle-of-the-road for nearby, ultracool dwarf stars. Kepler also didn’t reveal much activity, but it did catch at least one notable flare. Based on the spin and activity level, the authors estimate the star’s age is between 3 and 8 billion years.
Other M dwarf astronomers agree that that’s a reasonable range. Elisabeth Newton (MIT) says that most nearby stars are younger than 8 billion years. She and her colleagues recently surveyed nearly 400 nearby M dwarfs, finding that those with periods less than 10 days generally had ages of less than 2 billion years. But she cautions that the red dwarfs her team looked at were more massive than TRAPPIST-1, and the relationship between age and rotation period depends on the star’s mass. “I don’t think that the current data we have on the rotation periods of red dwarf stars is too useful for pinning down the ages of stars as small as TRAPPIST-1,” she warns.
John Bochanski (Rider University) agrees. TRAPPIST-1’s activity level implies that it’s not “really” old, he says, but beyond that it’s hard to say. It wouldn’t surprise him if the star was a little outside the range. Meanwhile, Jeffrey Linsky (University of Colorado, Boulder) puts his bet on 2 to 5 billion years, based on the star’s heavy-element content, X-ray output, and motion through the Milky Way.
Whatever the exact number, it’s likely that TRAPPIST-1 is about as old as the Sun. That permits all sorts of speculation about habitability and alien life, but given how much remains unknown about this system, I prefer not to dabble in such musings.
Reference: Rodrigo Luger et al. “A Seven-Planet Resonant Chain in TRAPPIST-1.” Nature Astronomy. May 22, 2017.
Visit the TRAPPIST-1 system with NASA's Visions of the Future poster.