A hot Jupiter in an extremely elliptical orbit
Every 111 days, the hot Jupiter HD 80606 b comes so close to its star that its surface temperature rises by 600 degrees Celsius. The large variations in distance result in distinct seasons – unlike on Earth, where they depend on the orientation of the axis of rotation relative to the Sun.
Exoplanets never cease to amaze us with their diversity: In the case of the hot Jupiter HD 80606 b, we encounter a hot gas giant with four times the mass of Jupiter, whose surface temperature varies by 600 degrees Celsius between winter and summer over the course of its 111-day year. It was discovered by a research team led by Tiffany Kataria at NASA’s Jet Propulsion Laboratory in Pasadena, California, using the James Webb Space Telescope (JWST).
Unlike Earth, where the seasons – and consequently the temperatures – depend on the position of the rotational axis, which is tilted at an angle of 23 degrees to the Sun, here the temperature fluctuations are caused by the exoplanet’s vastly varying distances from its host star. HD 80606 b orbits its star HD 80606 in a highly elliptical orbit with an eccentricity of e = 0.93, which is thus more reminiscent of long-period comets than of planets in our Solar System. When it reaches its furthest point from the star, the apastron, every 111 days, it is 133 million kilometres from the central star, which corresponds to 88 per cent of the distance between the Earth and the Sun. At the point in its orbit closest to the star, the periastron, the distance is just five million kilometres – or 3.1 per cent of the distance from Earth to the Sun. Due to the wide variation in its distance from the star, its surface temperature also varies extremely: when far from the star, it is around 550 degrees Celsius, but when close to the star, the temperature rises rapidly by around 600 degrees to approximately 1,200 degrees Celsius.
These temperature differences also influence the composition of HD 80606 b’s atmosphere. To analyse this, Kataria’s research team observed the exoplanet and its star over an extended period, including using the JWST. In doing so, they pointed the space telescope at the star before, during and after the planet’s passage through its periastron – including during the «secondary eclipse», when the exoplanet disappears behind HD 80606 as seen from our perspective. This required detailed planning, as this event can only be observed every 111 days and, from the JWST’s perspective, the star must not be too close to the Sun – a factor that depends on our planet’s orbit around the Sun. The analysis of the JWST data is still in its very early stages. However, even Kataria’s team was surprised by the severity of the temperature rise on HD 80606 b.
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