Joint work of ground and space observatories helped to discover a planet in the habitable zone of a very cold sta

(ORDO NEWS) — Two planets have been discovered around the nearby red dwarf LP 890-9, one of which is located in its habitable zone. The planets pass through the disk of the star, making them an interesting target for further research.

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 and brown dwarfs – and they are of considerable interest.

To have a temperate climate, the planet must be ten times closer to the red dwarf than the Earth is to the Sun, and in doing so, it will almost inevitably fall into a tidal lock.

On one side of it, turned to its luminary, like the Moon to the Earth, there will be eternal day, and on the other – eternal night, and only near the dividing line can a star rise from the horizon and set beyond it due to librations.

It is not easy for these planets to keep their atmospheres: red dwarfs have a frenzied flare activity, and in their youth they also have many times increased luminosity.

Joint work of ground and space observatories helped to discover a planet in the habitable zone of a very cold sta 2
Dependences of luminosity (vertically) on age (horizontally) for red and brown dwarfs. Both scales are logarithmic: an increase in one on the scale corresponds to a tenfold increase in the value. On the upper scale “-3” – one thousandth of the luminosity of the Sun, and on the lower “9” – the age of a billion years. The upper curve corresponds to a 0.10 solar mass red dwarf (slightly lighter than LP 890-9); the rest of the curves are brown dwarfs. The luminosity of such a star in the first few million years of its existence is almost a hundred times greater than its luminosity in adulthood

This is where the main difficulties of habitable planets in red dwarf systems end. If the planet is noticeably more massive than Earth, it can hold a dense atmosphere that will absorb the harsh radiation from the flares and smooth out the temperature contrasts between the day and night sides of the planet.

In addition, red dwarfs are several times more common than sun-like stars, and they live for hundreds of billions, or even trillions of years, unlike our Sun, which in a billion or two years will make the Earth uninhabitable.

Red dwarf planets are easier to study with transit spectroscopy, the flagship method for studying exoplanet atmospheres. Passing across the disk of a sun-like star, an Earth-sized planet covers one ten-thousandth of its disk, and its atmosphere covers only a couple of millionths.

If there is oxygen on the planet, then in the line of its absorption, the planet will cover approximately 0.0002% more starlight than in the neighboring part of the spectrum, where the atmosphere is transparent.

Joint work of ground and space observatories helped to discover a planet in the habitable zone of a very cold sta 3
An illustration of transit spectroscopy is the measurement of the fraction of a star’s light obscured by a planet as a function of absorption in its atmosphere. On the right is a graph of the change in observed brightness over time; “blue” waves are absorbed more strongly than “red” ones

Red dwarfs are much smaller than the Sun, and for their planets, the corresponding figures increase a hundred times – up to a percent and hundredths of a percent (they are inversely proportional to the square of the star’s radius).

Measuring such variations in brightness is still difficult, but much easier than for sun-like stars. In addition, due to close orbits, red dwarf planets are more likely to transit, and transit planets more often pass through the disk of their star.

Despite the convenience of studying, relatively few planetary systems are known in low-mass red dwarfs. Red dwarfs are very dim – especially in the visible range, where most observatories are working to search for exoplanets.

And yet, among their systems, they have already managed to make some impressive discoveries and compile interesting statistics.

The closest star to us, Proxima Centauri, is a red dwarf and has an Earth-mass planet in its habitable zone . Near the very dim and cold dwarf TRAPPIST-1 , seven Earth-like planets have been discovered at once, of which one to three may be in the habitable zone.

Many other red dwarf systems are built in a similar way: compact and orderly architectures of earths, super-Earths and mini-Neptunes, many of which are in or close to habitable zones.

Joint work of ground and space observatories helped to discover a planet in the habitable zone of a very cold sta 4
A planet in the Proxima Centauri system as seen by an artist. The planet LP 890-9 c, which is very similar to it, may look something like this

Scientists from the University of Birmingham, led by Laetitia Delrez, report the discovery of two Earth-like planets around the star LP 890-9, which now ranks second on the list of the coldest stars with planets (after TRAPPIST-1).

It is a red dwarf in the constellation Eridani, a hundred light-years from Earth, which is six times smaller than the Sun and nine times lighter than it.

The diameter of the star is only one and a half times that of Jupiter, and the temperature is less than that of a tungsten filament in many incandescent lamps – 2570 degrees Celsius.

In visible light, the brightness of LP 890-9 is +18th magnitude – you need a two-meter telescope to see it with your eyes. Despite this, the TESS observatory , which searches for exoplanets around nearby stars, has been able to record the dips in the brightness of a star from the passage of planets across its disk.

To confirm the transits, astronomers used the SPECULOOS meter-high infrared telescopes installed at the Canary and European Southern Observatories, and designed specifically to search for and confirm transits from red dwarfs (in the infrared they are hundreds of times brighter than in the visible).

The collecting power of these telescopes is a hundred times greater than TESS, and transit signals have been accurately measured.

It turned out that both planets have almost the same diameters – a third more than the earth. Most likely they are rocky. The inner planet is heated by the parent star four times more than the Earth, and has a thermal regime intermediate between Venus and Mercury.

On the outer planet, warming is 90% of the earth’s. Thus, the planet LP 890-9c is slightly cooler than the Earth and is located almost in the center of the habitable zone.

The transit signal from each of the planets – the drop in the brightness of a star when the planet passes through its disk – is a solid 0.6%. Probably, the James Webb telescope will soon join their study, and we will learn quite a lot about their nature and the composition of their atmospheres.


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