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A planetary system just 35 light years from Earth is home to four and possibly five planets. This includes one (if it exists) that is squarely in the habitable zone of its star, and another that is the lightest planet ever found using the radial velocity method: it has only 40% of the mass. of the earth. It’s pretty cool.
The host star is a red dwarf called L98-59, which is noticeably smaller, cooler, and darker than the Sun, shining only 1% of our own star’s brightness. Three planets were already known to orbit it, discovered using the mode of transport: Watch the star fade a fraction as a planet passes directly in front of it. This only works for systems that are almost exactly on edge as seen from Earth.
These planets were announced in 2019. Since then, a team of astronomers have re-observed the star using various telescopes to better characterize the planets. The transit method gives you the size of the planet (the bigger the planet, the darker the star, because the more its face is blocked), but we really need the mass to better understand the planets; with the mass you get the density (mass / volume) which tells you roughly what the planet is made of.
It takes the radial speed method: When the planet revolves around the star, its gravity pulls on the star. While the planet makes a large circle around the center of mass of the system, the star makes a smaller circle around it. This means that for half of the orbit the star is heading towards us and half away from us, and we can measure that using the Doppler shift.
This is extremely difficult to do, since the speed of the star is very low, but it is possible by dividing its light into a spectrum and carefully measuring the result. To further complicate matters, if there are multiple planets, there are multiple changes to each other.
However, the results are worth it. They could easily see the previously discovered planets (named L98-59b, c, and d, in order outward from the star), but they also found strong evidence of a fourth planet, called L98- 59th, and indices of a fifth.
The latter has yet to be confirmed, but if it is real, they find that it has a mass of at least 2.5 times that of the Earth and that it only takes 23 Earth days to orbit the Earth. star (the length of his year). If it is only slightly larger than Earth, it could be a rocky world. It orbits about 15 million kilometers from the star, only 1/10 of the distance from Earth to the Sun, but the star is so weak that it receives roughly the same amount of heat as Earth, so its temperature may in fact be close to ours. . It’s quite interesting! Unfortunately, it does not transit, so its size is unknown.
Again, if it exists, that would make it a very good candidate for it to resemble Earth. On the other hand, she might look like Venus if she has a thick atmosphere, so let’s not step forward. But stay.
The team also got much better measurements of the three inner planets, and the innermost, L98-59b, has a diameter of just 0.85 times the Earth and a mass only 0.4 times ours. This gives it a density of about 65% that of Earth. Considering its size, that means it’s rocky and might not have such a big iron core. It’s about 3.3 million kilometers from the star, so it’s hot, probably around 350 ° C (670 ° F). Ouch.
But it remains important! Measuring the mass of a planet using the radial speed method is easier the more massive the planet is; a strong man pulls harder on his star. A planet with only 0.4 Earth mass (or about half that of Venus) is extremely difficult to measure, and it is the lowest mass planet ever confirmed in this way!
This is good, because a planet like Earth orbiting farther from a star like the Sun transmits a slower speed to a closer planet, which also makes them hard to find. The results for L98-59b show that it is possible to take these measurements quite far.
Only half a dozen planetary systems have been found where we were able to get the diameters and masses of at least two planets, so it’s useful. Additionally, finding multiple planets with such different characteristics around a star is beneficial for people who study Comparative Planetology. Presumably these planets all formed around this star at the same time, which removes some uncertainty when compared to each other.
Also, I love those miniature red dwarf orbiting systems like TRAPPIST-1. They are so different from ours, with all the rocky planets so close to their host star. Yet, they may be the most common type of system in the Universe! If we are to find life there, despite our prejudices, the low-bulb red dwarfs may be the best places to look.
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