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The extraterrestrial worlds' chase into orbiting extraterrestrial stars continues: the NASA orbiting TESS observatory has just found a strange mini-Neptune in orbit around a star, and has also found tantalizing evidence of a world the size of the Earth! This last point has not been confirmed yet, but it is certainly worthwhile, if possible, to follow up.
TESS – the Transiting Exoplanet Survey survey satellite – launched in early 2018 and scanned the skies in the warm season millions stars to see if they have exoplanets orbiting around them. It uses the transit method, which means that it looks for a slight drop in brightness of a star if a) a planet gets to put it into orbit and 2) its orbit is near the edge of the Earth , so it goes directly in front of the face of the star (think of a mini-eclipse).
By carefully analyzing the hollows, one can determine the size of the planet as well as the orbital period (the year of the planet). Unlike Kepler, who has been looking at a point in the sky for years, TESS sweeps the sky over a period of two years, looking at the brightest stars. In general, brighter stars are closer to us and, because they are bright, it is easier for ground telescopes to follow up with more observations if needed.
Since the beginning of its operations, TESS has found nine confirmed exoplanets and we can now add at least one.
The star in question is called HD 21749 (the 21749th Star catalog Henry Draper). It is a star very similar to the Sun, but smaller (70% of the diameter), less massive (0.73 times the mass of the Sun), colder (4 640 K against 5 800 K of the Sun) and less bright (it shines with only 20% of the brightness of the sun). It's pretty close too: only 53 light-years away. This is the 8.1 magnitude, which means that you will need binoculars to see if you have a clear view of the southern constellation, the reticulum.
The planet found, called HD 21749b, has an orbit about 30 million kilometers from the star (the Earth revolves around the Sun five times farther) and takes a little less than 36 days to orbit it once – that's it. is the longest planet that has been found. to date with TESS; it moves quite often from one sky to another, so it is difficult to find planets with this long time. Because the star is colder than the sun, it means that the temperature of the planet is probably about 150 ° C, warmer than the boiling point of the water. So, do not plan a vacation there.
Not that it would be fun otherwise. The planet is about 2.8 times the diameter of the Earth. TESS can not find the mass of a planet, but the astronomers who found this planet searched for star observations in the archived data and found that HARPS – the high-precision planet searcher at radial velocity – l had observed several times. HARPS works by dividing light into extremely fine wavelengths (colors); When a planet revolves around a star, it shoots the star, so that the star makes a small orbit around the center of mass planet / star. This gives the light a small Doppler shift which, once measured, gives the mass of the planet.
The signature of the HD 21749b was clearly visible in the light of the star. They discovered that its mass was about 23 times that of the Earth. It's interesting! Given its size, this gives it a density of 5.7 grams per cubic centimeter, about the same density as the Earth. For a smaller planet, I would say that it means that its composition is similar to that of Earth: rock and metal. But this planet being much more massive, its atmosphere is probably thick.
I must admit it's weird. This planet is more massive than Neptune (about 17 times the mass of the Earth) but smaller (Neptune is 3.9 times the diameter of the Earth). So, if it has a thick atmosphere like Neptune, the core needs to be really dense to compensate.
The second planet is less constrained. They witnessed a very small dip in the light of the star with a period of 7.8 days, which corresponds to a planet of about 90% of the diameter of the Earth (rather to Venus, in reality). This period puts about 10 million kilometers outside, so it is very hot, more than 400 ° C! It's hot enough to melt tin, zinc and lead, so yuk.
However, it is not yet possible to confirm the existence of the planet. The mini-Neptune has been confirmed in the data with the help of HARPS, but this candidate planet is too small a mass to be seen in these data. It would be a challenge to see him in all those data. The star is bright enough that dedicated tracking observations with some ground telescopes can be found, but since the light drop is less than 0.02%, it's, uh, difficult. So, it may take some time before this candidate is confirmed.
Another problem is that a bright star located very close to HD21749 in the sky could interfere with the measurements of the second planet (the first mini-planet Neptune is not affected by the star because its signature is visible from two different ways (observations). It is possible that this planet does not exist at all! The data seems pretty convincing, so hopefully, but for now, there's no way to be sure. I will note that if is confirmed, it is the smallest planet that TESS has found yet.
In the meantime, TESS is still watching millions of other stars. One of its main objectives is to find 50 exoplanets in transit less than 4 times the diameter of the Earth with measured masses. HD 21749b meets these criteria, so it's at least one more fluff!
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