Our cosmic neighborhood seems to be teeming with exoplanets, even our nearest neighbor is orbited by three planetary celestial bodies. Now astronomers have discovered another nearby star with two super-Earths. They orbit the bright red dwarf just 33 light-years away, making them the fourth-closest multiplanet system observable by the transit method. With temperatures of a good 400 and almost 300 degrees, both exoplanets are too hot for life. For this, the planetary system could be one of the most suitable targets for closer observations by the new James Webb Space Telescope.
Our solar system is not alone in space, but is surrounded by countless other stars and planetary systems. Within a radius of 33 light-years alone there are more than 400 red dwarfs and sun-like dwarfs, many of which have multiple exoplanets. Our nearest neighboring star Proxima Centauri has three planets, the red dwarf Wolf 1061, which is only eleven light years away, also has three, one of which is a super-Earth in the habitable zone. About 40 light-years away, the star Trappist-1 is orbited by seven exoplanets, several of which could have potentially life-friendly temperatures. What conditions actually prevail on such nearby exoplanets and what their atmospheres are like – if they have any – could soon be clarified for the first time by the James Webb Space Telescope. Because only its infrared optics are powerful enough to decode the spectral signal of such exoplanetary atmospheres with high resolution.
Nearby red dwarf in sight
Now an international team of astronomers led by Rafael Luque from the Institute for Astrophysics in Granada, Spain, has discovered a promising newcomer among the nearby stars with planets. Analyzing data from NASA’s Transiting Exoplanet Survey Satellite (TESS), the scientists encountered anomalies in the light curve of the bright red dwarf star HD 260655, 33 light-years away. Periodic shadowing of the starlight indicated that this star had two planets passing in front of it could own. To rule out that this periodic dimming of the star was caused by its rotation or stellar activity, Luque searched the archives of several ground-based telescopes for observational data of this red dwarf.
In fact, several telescopes had targeted the star over the past 20 or so years. A reanalysis of some of this data confirmed that the red dwarf has a rotation period of 30 days and little activity. The temporal pattern of both parameters did not match the periodicity of the shadowing observed with TESS, which occurs at an interval of 2.7 and 5.7 days. “We do not find signals that correspond to the transit periods of these two potential planets,” the astronomers write. To find out more about these two celestial bodies, the researchers used data from other telescopes to draw conclusions about the mass and composition of the two potential exoplanets via the radial velocity. In this method, astronomers use tiny shifts in the light spectrum to detect a star’s slight “wobble” caused by the gravitational forces of the planets orbiting it.
Two super earths with different densities
The analyzes confirmed that the nearby red dwarf HD 260655 must have two planets. The inner one, HD 260655 b, orbits its star very closely in just 2.7 days, is about twice Earth’s mass and 1.2 times its size. From this, the astronomers conclude that it could be a rocky super-Earth. “260655 b has a density that perfectly matches that of Earth,” Luque and his team write. However, with an estimated temperature of 435 degrees, this exoplanet would not be very Earth-like given the conditions on its surface and would be too hot for life. The second exoplanet, HD 260655 c, orbits further out with an orbital period of 5.7 days and is about 1.5 times the size of Earth. At three times the mass of Earth, this planet is a super-Earth in size, but significantly less dense than a classic terrestrial planet with an iron core and rocky mantle.
According to the researchers, there are two possible explanations for this: HD 260655 c could be a rocky planet without an iron core, which does not fit well with current planet formation models. Or the exoplanet appears larger in relation to its mass because it has an atmosphere filled with water vapor. “If one were to assume an Earth-like composition for HD 260655 c, then a water content of just one percent could explain the differences in density,” the team says. Where this water comes from is still an open question given the close proximity of even this outer planet to its star. Theoretically, it could have formed further out in the accretion disk of its star and then migrated inward afterwards. However, it is also conceivable that it received the water in the form of ice from the impact of comet-like chunks in the early days of this system.
NASA’s James Webb Space Telescope could soon help clarify whether the super-Earth HD 260655 c has an atmosphere and how these two, on the one hand similar, on the other hand different, two worlds formed around the red dwarf HD 260655. This will send the first scientific recording to Earth in mid-July and thus start its scientific operation. In addition to observing distant, early galaxies, one of the main tasks of this telescope is to target the atmospheres of exoplanets using its high-resolution infrared spectroscopes. “Due to the relatively high apparent brightness of the host star, the newly discovered planets are excellent targets for further atmospheric studies,” explains co-author Karan Molaverdikhani from the University Observatory of the Ludwig Maximilian University in Munich. These studies could allow astronomers to gain more detailed information about the composition and structure of the atmospheres of both planets around HD 260655, and also valuable insight into the factors shaping the evolution of Earth-like planets in general.
Source: Rafael Luque (Instituto de Astrofísica de Andalucía, Granada) et al., Astronomy & Astrophysics, accepted; doi: 10.48550/arXiv.2204.10261