An unusual planetary dance has produced an Earth-sized exoplanet brimming with volcanoes, likely furnished with an atmosphere, and maybe even containing water on its surface.
Astronomers discovered this odd world orbiting a red dwarf star right in our galactic neighborhood—just 90 light-years away. The exoplanet, known as LP 791-18 d, orbits a red dwarf already known to host two other planets: LP 791-18 b, a scorched, rocky world orbiting extremely close to the star, and LP 791-18 c, a sub-Neptune 7 times more massive than Earth made of gas or icy material.
The finding comes after a group of researchers led by astrophysicist Björn Benneke of the Université de Montréal in Canada used NASA’s recently retired Spitzer Space Telescope to take a closer look at the system. After staring at it for five consecutive days, Spitzer detected a new Earth-sized planet crossing in front of the star—transiting, in astronomical lingo.
“There were several months we were worried that we couldn’t confirm that the planet was even real.”
At least two transits are necessary to determine a planet’s orbital period and gauge its mass and density, so Benneke and his colleagues monitored the planet with several ground-based observatories. But subsequent transits never arrived on time.
“One of the challenges, and this is why we can also now say that there is volcanism, is that we couldn’t find the planet at the time that we predicted,” Benneke said. “There were several months we were worried that we couldn’t confirm that the planet was even real.”
Eventually, they unraveled the mystery. LP 791-18 d is nestled between the system’s two other planets and has an elliptical orbit that takes it very close to the outer and larger planet (LP 791-18 c). At their closest approach, the planets are only 1.5 million kilometers apart, about 4 times the distance between the Earth and the Moon. Every rendezvous, planet c gives its smaller sibling a gravitational tug that further elongates its orbit, scrambling the timing of the smaller planet’s transits in the process. The researchers reported their findings in Nature.
Hot Inside
LP 791-18 d is a rocky planet with a mass and density similar to those of Earth. As it travels along its elliptical orbit, which takes an average of 66 hours, its distance to the star varies slightly. As it gets closer, a gravitational squeeze deforms the planet, creating internal friction and heating things up from the inside, a process called tidal heating.
“It’s a bit like when you take Play-Doh and you squeeze it continuously; at some point it gets warm,” Benneke said. “That heat eventually wants to get to the surface, and the only way rocky planets can do that is through volcanism.”
The researchers calculate that tidal heating alone creates a heat flow from LP 791-18 d’s interior to its surface 2 to 10 times larger than Earth’s. Our planet’s heat is a remnant of its formation, aided by the decay of radioactive elements in the mantle. Because these phenomena are likely also present on LP 791-18 d, it is easy to imagine that its volcanism is much more intense.
“I think this is a remarkable discovery,” said Amy Barr Mlinar, a planetary scientist at the Planetary Science Institute and chief academic officer at the Summer Science Program, who wasn’t involved in the study. “Since the discovery of TRAPPIST-1, we’ve been looking for other systems that might have similar behavior, and in order to have tidal heating happen over a geologically long timescale you need multiple planets orbiting the same system.”
A Volcanic Atmosphere
In addition to volcanoes, LP 791-18 d likely has an atmosphere. Volcanism can release volatiles trapped in the mantle, which build up over time and envelop the planet in a gaseous layer.
Stellar radiation, however, can erode this atmosphere away. Red dwarfs are temperamental stars—they flare often and emit powerful stellar winds that strip atoms away from the upper layers of the atmosphere. Because planetary systems around red dwarfs are tightly packed—the whole LP 791-18 system is 10 times smaller than the orbit of Mercury—the effect of stellar radiation is more pronounced.
“This is a huge opportunity to understand the diversity of rocky planets around other stars.”
Even so, researchers are optimistic about the possibility of detecting an atmosphere around LP 791-18 d. The volcanic activity can potentially replenish the atmosphere as quickly as it’s lost. “That actually makes it much more likely that there’s an atmosphere that we can detect,” Benneke explained.
The James Webb Space Telescope (JWST) should be able to characterize such an atmosphere, revealing a lot about the type of volcanism that occurs on the planet, its internal composition, and how sensitive it is to atmospheric loss. “This is a huge opportunity to understand the diversity of rocky planets around other stars,” Benneke added.
Habitability Boom or Bust
In addition to contributing to LP 791-18 d’s volcanic activity, tidal heating could potentially bend the limits of its system’s habitable zone—the just-right distance from a star in which a planet can retain liquid water. The habitable zone is traditionally defined by the radiation coming from the star and how much it can heat a planet but does not account for tidal heating.
“This particular system is really unique,” said Juliette Becker, an astrophysicist at the California Institute of Technology who wasn’t involved in the study. Tidal effects decay very strongly with the distance to the central body, Becker explained, so planets that are close enough to a star to feel its effects are already too hot for habitability. Because red dwarfs are cool stars, however, “you can have potentially habitable planets that have significant tidal interior heating, so it’s a kind of unique place to be,” she said.
Ultimately, she said, “tidal heating can help, or hurt, or not affect at all the habitability.” For instance, tidal heating can benefit habitability by adding an atmosphere to a planet that would otherwise be barren or maintain a subsurface ocean on a planet otherwise too cold for liquid water.
“On Earth, we have organisms that live near hydrothermal vents that are powered by heat from the interior,” Becker said. “You can imagine other planets that have more extreme conditions combined with water oceans having the potential for extremophiles to develop.”
Tidal heating can also do just the opposite. If volcanism is too strong, constant resurfacing of the planet could potentially destroy emerging life or prevent it from developing in the first place. The planet could also end up with an extremely thick atmosphere with no way to recycle volatiles into the mantle, a job performed by plate tectonics on our own planet.
Tidal heating could also drive runaway greenhouse heating. If the internal heat flux is higher than the planet’s capacity to radiate heat to space, the surface can get extremely hot, evaporating any surface water, which could then be lost to space.
It’s also unclear how extreme tidal heating can affect a planet’s global magnetic field. Some studies suggest that it can actually suppress a planet’s internal dynamo, killing the magnetic field and increasing atmospheric loss.
Benneke and colleagues are on the fence about the effects of tidal heating on the habitability of LP 791-18 d. On the one hand, enhanced volcanism can provide an atmosphere that would otherwise be lost to stellar radiation. At the same time, the planet is tidally locked, with the same side always facing the star. Although on the dayside temperatures could reach 600 K, the nightside is much cooler, and water, if present, could condense and form ice. Under it, volcanism could form a subsurface liquid layer and provide nutrients for life to develop. In the twilight zone between the two sides—the terminator zone—moderate temperatures could occur.
Benneke and his team are already making a proposal to the JWST to take a closer look at LP 791-18 d at some point next year, which could answer some of their questions. “This is a superexciting target to look at, so I’m pretty sure that this is going to happen,” Benneke said.
—Javier Barbuzano (@javibarbuzano), Science Writer