Planetary Sciences News

Seven Earth-Sized Planets Seen Whizzing Around One Cool Star

Although all the exoplanets orbit closer than Mercury does to our Sun, liquid water may persist on some of them because their star radiates so little heat.

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Forty light years away, seven Earth-sized planets circle a small, dim, “ultracool” dwarf star, some completing their orbits as fast as three or four times per Earth month. These exoplanets, four of them previously unknown, “are the best targets so far to search for signs of life” outside our solar system, said Julien de Wit, a planetary scientist at the Massachusetts Institute of Technology in Cambridge and coauthor of a paper about the planets published today in Nature.

All of these remote worlds orbit close to their dim star, meaning that enough energy may reach the outermost planets to provide liquid water. In addition, the planets’ orbits do not dip very much below or above the plane of the star, which would allow for the star’s gravity to push and pull the planets enough to perhaps produce internal heat. There might even be enough internal heat to generate strong volcanic activity on some of the planets, de Wit said.

The seven planets serve as “true Rosetta Stones” for studying exoplanets, de Wit added. They offer observers a “winning combination” of revealing information because they regularly pass between the star and Earth, throwing doors wide open for detailed atmospheric studies, he continued.

The Transit Method

To find the seven bodies, the team used the most common method by which scientists find exoplanets: the transit method. The scientists observed the star, called TRAPPIST-1, via ground-based and space-based telescopes, looking for dips in its brightness. A periodic dip in the star’s brightness means that something moves between the star and Earth on a regular basis, like a planet.

Recently, exoplanet hunters have focused attention away from observing stars like our Sun—large and bright—to the smallest, coldest stars to look for planets. Michaël Gillion, lead author of the new paper, said that this is because the light from Sun-like stars often drowns out any signal from small, rocky, Earth-sized planets. Instead, many scientists began to wonder: Why not look where our current technology can clearly see? This would be around small, ultracool dwarf stars like TRAPPIST-1, which is one ninth the diameter of our Sun and only half as bright.

Following this logic, the researchers used the ground-based Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the European Southern Observatory’s La Silla site in Chile. Last year, the researchers spotted the three outermost planets but suspected there were more because previous exoplanet discoveries from the Kepler mission “show that multiplanet systems are very common,” said Katherine Deck, an astronomer at the California Institute of Technology in Pasadena and coauthor of the paper.

So the researchers turned to the Spitzer Space Telescope, which observes the solar system in infrared light—light with wavelengths longer than the human eye can see. For 20 days, Spitzer observed the TRAPPIST-1 system, watching the star’s brightness decline, then rise again as planets passed by.

Combining the Spitzer data with more ground-based measurements, the researchers observed 34 different transits, from which they teased out not three planets but seven. Although the team hasn’t defined the orbital period of the outermost planet, the orbits of the other six range from 1.5 to 12.7 Earth days. Because the star is so small, “the signal of TRAPPIST-1’s planets is about 80 times larger than what it would be if they were orbiting our Sun,” de Wit said. Only Jupiter passing in front of the Sun could produce as pronounced a dip in solar intensity for an observer watching our solar system from a similar distance, the researchers noted.

The transits provided researchers with diameter estimates, calculated from the degree to which the starlight diminished as the planet interceded between the star and Earth. Larger-diameter bodies obscure more of the stellar surface, causing proportionately more dimming.

The researchers also looked at how the planets gravitationally tugged on each other as they orbited TRAPPIST-1, which “causes the timing of their transits to change a little,” de Wit said. Sometimes the transits come early, sometimes late. By measuring these variations, the researchers can start to determine the masses of the planets. In a year or so, he continued, they should be able to constrain the masses fairly precisely.

Artist’s representation of the seven planets in the TRAPPIST-1 system.
An artist’s representation of the seven TRAPPIST planets, based on new measurements of their masses and diameters. Credit: NASA/JPL-Caltech

Search for Life

Because of the proximity of the planets to their star, the team suspects that the innermost planets exhibit a runaway greenhouse scenario, much like Venus. This means that liquid water is unlikely to be sustained on their surfaces.

The outer three planets, however, could orbit far enough from their star to harbor liquid water. To investigate further, the researchers plan to take a closer look at the exoplanets’ atmospheres. Researchers investigate exoplanet atmospheres by examining patterns in the star’s light as it passes through the atmosphere. If the planet has an envelope of gases surrounding it, the light’s signature will indicate that.

Researchers note that when the James Webb Space Telescope launches next year, they will have an unprecedented opportunity to observe these seven new atmospheres to search for potential biological signatures such as methane, ozone, and carbon dioxide. Already, the Hubble Space Telescope has determined that two of the TRAPPIST-1 planets probably do not host hydrogen- and helium-dominated atmospheres—if they did, it would have barred habitability.

“We already knew that Earth-like exoplanets are common in the Milky Way, but this new finding suggests that they are even more abundant,” said Ignas Snellen, an astronomer at the Leiden Observatory at Leiden University in the Netherlands who wasn’t involved in the new research. The new research shows that “in our search for planets like Earth and possible extraterrestrial life, it really pays to concentrate on the smallest stars.”

—JoAnna Wendel (@JoAnnaScience), Staff Writer

Citation: Wendel, J. (2017), Seven Earth-sized planets seen whizzing around one cool star, Eos, 98, https://doi.org/10.1029/2017EO068531. Published on 22 February 2017.
© 2017. The authors. CC BY-NC-ND 3.0