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Now that astronomers have bagged more than 4,000 exoplanets – extraterrestrial worlds orbiting extraterrestrial stars – we are starting to see some of them that are really strange. Massive planets approach their stars, planets with two or even three stars, or even moons.
And some who are not just strange. They are eccentric.
I mean that literally. Eccentricity is an astronomical term indicating how much the orbit of an object is different from a circle. It's originally an old mathematical term for describing an ellipse. An eccentricity of 0 is a perfect circle and the ellipticity increases as the eccentricity increases (up to a value of 1, at exactly 1, you have a parabola and, if it is> 1, a hyperbola) .
The eccentricity of the Earth's orbit is <0.02. The most eccentric planet in our solar system is Mercury, with an eccentricity of 0.205; even the wildly eccentric orbit of Pluto has only a value of 0.244.
Astronomers have just discovered an exoplanet with an orbital eccentricity of 0.84. Holy wow. It is as close to its parent star as it is about 440 million kilometers (roughly where the asteroid belt is in our solar system) and up to 5 km. billion kilometers – further from his star than Neptune gets sun!
The planet is called HR 5138 b. Its host star (HR 5138, of course) is a bit like the sun, actually. It's a star G0, so it's only a little bigger than the Sun and about 1.5 times the Sun's mass. Interestingly, it is old, about 7.7 billion years old, three billion years older than the Sun. It's just a hair more than 100 light-years away, and maybe it's just barely visible to the naked eye in the constellation of the Virgin if you look at it from an extremely dark site without a moon (its magnitude is 6.3 if you like the technical characteristics).
Astronomers who have discovered the planet have been monitoring the HR 5183 for 22 years (!) Using various telescopes, including the 10 meter Keck, a 2.7 meter telescope at the McDonald Observatory in Texas and 2, 4 meters to Lick Observatory. . They used high resolution spectra to search for a planet using the reflex speed method.
In summary, a planet revolves around a star because of the gravity of the star. But the planet also has gravity, so that the star moves in response to the movement of the planet, we call it reflex speed. The amount of change in star speed depends on the mass of the planet and its proximity, but the way the speed of the star changes with time also depends on the shape of the planet's orbit. A circular orbit gives a nice sinusoidal curve to the speed of the other star, but if the planet's orbit is really eccentric, you'll see very little change for a long time when the planet is far away, then a rapid increase when the planet lowers and then passes in front of the star.
All this is measured by looking at the spectrum of the star, its light divided into very narrow colors. When the star moves towards us, the spectrum turns into a blues shift, and when it moves away, it turns into a redshift. Normally, these offsets are small and the spectrum is noisy, but this is not the case for HR 5183.
Yuck! This huge spike is the classic reflex speed of a very eccentric planet. The star has a normal speed towards the Sun of a few kilometers per second (from its movement around the galaxy). Then, in 2018, yowza. When they saw the sudden increase in the speed of the star, they began to take more observations and got a lot of information on the planet when it overtook l & # 39; star.
The minimum mass of the planet is a little more than 3 times the mass of Jupiter. It could be more; the angle at which we see the orbit can affect the speed of view (if we see the face of the orbit face to face, reflex velocity is measured at 0, and if we see it at edge, it is maximized, there is a trigonometric sinusoid dependence turn affects the measured mass). The orbital period (the year) of the planet is about 74 years old, although there is considerable uncertainty about it (it could reasonably be between 50 and 120 years old). The shape of the curve is what gives the eccentricity of the orbit, and that's how they found it was 0.84.
The next question is: why is the orbit so elongated? It is likely that another massive planet orbiting the star is too close together and that their mutual severity has mutually affected each other. Astronomers have searched and found no other massive planet in the data, however. Sometimes the encounter between two planets can be so intense that one of the planets is ejected from the system, and I strongly suspect that this is what happened here. It is possible that something like this happened in our own solar system 4 billion years ago!
There is another nearby star, the HR 5182, an orange dwarf of lower mass, but it is 2.2 trillion kilometers away, almost a quarter of a light year! The HR 5183 may be linked (in other words, the two orbits around an extremely long time scale), but it is far too far away to affect the planet.
They only caught this planet because they were watching it at the periastron approach, at the closest approach to the host star. Most of the time, it's too far to see any effect. However, it is possible that when the planet moves away from the star, it is visible by direct imagery. Given the long orbit, for decades at least, it will take time before we can do it. But it's pretty cool.
It is also possible that its effect on the star is visible in the Gaia Observatory data, which measures the movements of stars in the sky. When the planet moves around the star, the star itself physically moves. it's a very small amount, but it might be possible to see it in the Gaia data when the next big release arrives in a few years. I will have to keep my eyes open for that.
I dig this. As we discover more exoplanets, we find others that stand out from the crowd. From an observational point of view, such observations are interesting because they make it possible to bridge the gap between the planets visible in the RV data and those visible directly in the images. Physically, it's interesting because the really eccentric planets have unusual original stories, and learning about them places our own solar system in context.
Plus, come on, it's just cool. I like that too.
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