Discovery of a planet in the habitable zone

Two planets have been discovered around the nearby red dwarf LP 890-9, one of which is located in its habitable zone. These planets cross the star's disk, making them interesting targets for further study.

When we talk about habitable planets, we usually think of a second Earth orbiting a second Sun. But habitable planets can also orbit cooler luminaries - red dwarfs and brown dwarfs - that are of considerable interest.

To have a mild climate, the planet would have to be ten times as far from the red dwarf as the Earth is from the Sun, and doing so would almost inevitably lead to a tidal lock. On one side of it, turned toward its luminaries, as the Moon is to the Earth, there would be eternal daylight, and on the other side - eternal darkness, only near the dividing line, a star could rise from the horizon and cross it. To the great joy. It is not easy for these planets to keep their atmospheres: red dwarfs have crazy flare activity and their luminosity increases many times during their youth.

The dependence of the luminosity (vertical) on the age (horizontal) of red and brown dwarfs. Both scales are logarithmic: an increase of one on the scale corresponds to a tenfold increase in the value. The upper "-3" - thousandth of the solar luminosity, the lower "9" - billion years of age. The upper curve corresponds to a red dwarf of 0.10 solar masses (slightly lighter than LP 890-9); the rest of the curve is for brown dwarfs. The luminosity of such stars in the first few million years of their existence is almost a hundred times their luminosity in adulthood.

This is where the main difficulties of habitable planets in red dwarf systems end. If the planet is significantly more massive than the Earth, it can have a dense atmosphere that absorbs hard radiation from flares and eliminates the difference in temperature between the day and night of the planet. In addition, red dwarfs are several times more numerous than Sun-like stars, and they live hundreds of billions, if not trillions, of years, unlike our Sun, which would make Earth uninhabitable in another billion or two years.

Red dwarfs are easier to study with lunar spectroscopy, the flagship method for studying exoplanet atmospheres. Across the disk of a Sun-like star, an Earth-sized planet covers a thousandth of its disk, while its atmosphere covers only a millionth. If there is oxygen on the planet, the planet will cover about 0.0002% more starlight in its absorption lines than in the neighboring part of the spectrum (atmospheric transparency).

Red dwarfs are much smaller than the Sun, and for their planets, the corresponding numbers increase by a factor of a hundred - up to one and a half percent (they are inversely proportional to the square of the stellar radius). It is still difficult to measure this brightness variation, but it is much easier than measuring a Sun-like star. In addition, because of their close orbits, red dwarfs are more likely to overpass the Sun, and overriding planets cross the disk of their stars more frequently.

Despite the ease of study, there are relatively few known planetary systems in low-mass red dwarfs. Red dwarfs are very faint - especially in the visible range - and most observatories are struggling to find exoplanets. In their systems, however, they have managed to make some impressive discoveries and compile interesting statistics.

Our nearest star, Proximal, is a red dwarf with an Earth-mass planet in its habitable zone. Near the very faint and cool dwarf TRAPPIST-1, seven Earth-like planets have been discovered at once, one to three of which may be in the habitable zone. Many other red dwarf systems are built similarly: the compact and ordered structures of Earth, super-Earths, and mini-Neptune, many of which are in or near the habitable zone.

Scientists at the University of Birmingham, led by Letitia Delores, report the discovery of two Earth-like planets around the star LP 890-9, which now ranks second (after TRAPPIST-1) on the list of coolest stars with planets. It is a red dwarf in the constellation Poe, one hundred light-years from Earth, six times smaller and nine times lighter than the Sun. The star is only one and a half times larger in diameter than Jupiter and has a temperature below that of the tungsten filament in many incandescent lamps - 2570 degrees Celsius.

In visible light, the brightness of LP 890-9 is +18 mag - you need a two-meter telescope to see it with your eyes. Still, the TESS observatory, which searches for exoplanets around nearby stars, was able to record the drop in the star's brightness as the planet crossed its disk.

To confirm the transits, astronomers use the SPECULATORS meter-high infrared telescopes installed at the Canary and European Southern Observatories, dedicated to searching for and confirming transits of red dwarfs (in the infrared, they are hundreds of times brighter than visible light). These telescopes have a collection capacity a hundred times that of TESS and have accurately measured the transiting signal.

It turns out that the two planets are almost the same diameter - a third larger than the Earth. They are most likely rocky. The inner planets are four times hotter than the Earth by their parent star, and their thermal state is somewhere between Venus and Mercury. On the outer planets, the warming is 90% of that of the Earth. Thus, planet LP 890-9 is slightly cooler than Earth and lies almost in the center of the habitable zone.

The sunset signal from each planet - the decrease in the brightness of the star as the planet passes its disk - is a steady 0.6%. Perhaps the James Webb Telescope will soon join their study and we will learn a lot about its nature and atmospheric composition.