Newfound super-Earth alien planet whips around its star every 0.67 days

We keep getting reminders that the Milky Way's planetary diversity dwarfs what we see in our own solar system. The newfound exoplanet TOI-1685 b is yet another case in point. Astronomers found it circling a dim red dwarf star about 122 light-years from Earth. "Circling" is too ordinary a world for TOI-1685 b's motion, however; the alien world whips around its parent star once every 0.67 Earth days. Red dwarfs, also known as M dwarfs, are much smaller and dimmer than Earth's sun, but TOI-1685 b's extreme proximity to its host star, called TOI-1685, makes it a very toasty world nonetheless. The discovery team estimates its surface temperature to be around 1,465 degrees Fahrenheit (796 degrees Celsius). The researchers, led by Paz Bluhm of Heidelberg University in Germany, first spotted TOI-1685 b in observations made by NASA's Transiting Exoplanet Survey Satellite (TESS). As its name suggests, TESS looks for transits, the tiny brightness dips caused by planets crossing their host stars' faces from the Earth-orbiting spacecraft's perspective. TESS noted such a dip around the red dwarf TOI-1685. Bluhm and her colleagues then confirmed the planet's existence using data gathered by the CARMENES spectrograph instrument, which is installed on the 3.5-meter telescope at the Calar Alto Observatory in Spain. (CARMENES is short for "Calar Alto high-Resolution search for M dwarfs with Exo-earths with Near-infrared and optical Echelle spectrographs.)

CARMENES hunts for planets using the radial velocity, or Doppler, method — looking for little wobbles in a star's motion caused by the gravitational tug of an orbiting planet.

toi-1685b

The combined data allowed the team to determine that TOI-1685 b is a "super-Earth" about 1.7 times bigger, and 3.8 times more massive, than our home planet. The resulting bulk density — about 4.2 grams per cubic centimeter, or 0.15 lbs. per cubic inch — makes TOI-1685 b "the least dense ultra-short period planet around an M dwarf known to date," Bluhm and her colleagues wrote in the discovery paper, which you can read for free on the online preprint site arXiv.org. (The paper has not yet been published in a peer-reviewed journal.)

For perspective: Earth's bulk density is about 5.5 grams per cubic centimeter, or 0.20 lbs. per cubic inch.

The fact that TOI-1685 b transits and is quite warm makes it a good candidate for follow-up study by other instruments, the researchers wrote. In that regard, TOI-1685 b is similar to another recent exoplanet find made using TESS and CARMENES data, Gliese 486 b.

Bluhm and her team also saw another signal in the CARMENES TOI-1685 data, which could indicate a second planet in the system that orbits once every nine Earth days. If this candidate planet exists, it doesn't transit, because TESS recorded no corresponding signal, the researchers wrote.

A planet that should not exist

Astronomers detected a giant planet orbiting a small star. The planet has much more mass than theoretical models predict. While this surprising discovery was made by a Spanish-German team at an observatory in southern Spain, researchers at the University of Bern studied how the mysterious exoplanet might have formed. The red dwarf GJ 3512 is located 30 light-years from us. Although the star is only about a tenth of the mass of the Sun, it possesses a giant planet - an unexpected observation. "Around such stars there should only be planets the size of the Earth or somewhat more massive super-Earths," says Christoph Mordasini, professor at the University of Bern and member of the National Centre of Competence in Research (NCCR) PlanetS: "GJ 3512b, however, is a giant planet with a mass about half as big as the one of Jupiter, and thus at least one order of magnitude more massive than the planets predicted by theoretical models for such small stars." The mysterious planet was detected by a Spanish-German research consortium called CARMENES, which has set itself the goal of discovering planets around the smallest stars. For this purpose, the consortium built a new instrument, which was installed at the Calar Alto Observatory at 2100 meters altitude in southern Spain. Observations with this infrared spectrograph showed that the small star regularly moved towards and away from us - a phenomenon triggered by a companion who had to be particularly massive in this case.

Because this discovery was so unexpected, the consortium contacted, among others, the Bern research group of Mordasini, one of the world's leading experts in the theory of planet formation, to discuss plausible formation scenarios for the giant exoplanet. The paper with all contributions has now been published in the journal Science.

Bottom-Up Process or Collapse?

"Our model of the formation and evolution of planets predicts that around small stars a large number of small planets will be formed," Mordasini summarizes, referring to another well-known planetary system as an example: Trappist-1. This star comparable to GJ 3512 has seven planets with masses roughly equal to or even less than the mass of the Earth. In this case, the calculations of the Bern model agree well with the observation. Not so with GJ 3512. "Our model predicts that there should be no giant planets around such stars," says Mordasini.

One possible explanation for the failure of current theory could be the mechanism underlying the model, known as core accretion. Planets are formed by the gradual growth of small bodies into ever larger masses. The experts call this a "bottom-up process."

Maybe the giant planet GJ 3512b was formed by a fundamentally different mechanism, a so-called gravitational collapse.

"A part of the gas disk in which the planets are formed collapses directly under its own gravitational force," explains Mordasini: "A top-down process." But even this explanation poses problems. "Why hasn't the planet continued to grow and migrate closer to the star in this case? You would expect both if the gas disk had enough mass to become unstable under its gravity," says the expert and adds: "The planet GJ 3512b is therefore an important discovery that should improve our understanding of how planets form around such stars."