A research team from Keele University in the UK has described one of the darkest planets ever seen. In their paper uploaded to the arXiv preprint server, the team describes the planet and where it appears to be under other dark planets.
Planet WASP-104b in question is a hot Jupiter, a gas giant that passes very close to its star – about 4.3 kilometers (it circles its star every 1.75 days). It is considered dark because its atmosphere absorbs about 97 to 99 percent of the visible light that hits it from its star. WASP-104b is also locked, which means that one side always faces the star, while the other side is colder and darker.
The researchers suggest that the reason the planet is so dark is probably because it is locked. Such a situation, it is believed, would cause the side facing its star (a yellow dwarf) to be too hot for cloud or ice formation, which normally brightens a planet by reflecting light. They suspect that the planet also has a very thick atmosphere that absorbs light. The atmosphere is probably potassium and atomic sodium – both absorb most of the light in the visible spectrum. But because the researchers can not really see the planet, they have to guess what it actually looks like – probably a glowing purple glow, they suggest.
Planets like WASP-104b are usually not found by direct observation through a telescope, but by the transit method – by detecting and measuring how much a star obscures and how long a planet passes over it. With planets as big as WASP-104b, researchers can use the radial velocity method, where a star shakes slightly due to the gravity of a planet orbiting it.
WASP-104b is not the darkest planet that was discovered – the record holder is TrES-2b, from which earlier investigations have shown reflects only 0.1 percent of the incident light. WASP-104b is probably in the top three, though researchers estimate that due to the inability to gather accurate measurements, it's difficult to say which planets are really darker than others.
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WASP-104b is darker than coal, arXiv: 1804.05334 [astro-ph.EP] arxiv.org/abs/1804.05334
An analysis of the K2 short cadence data from Campaign 14 detects the phase modulation in the light curve of the Hot Jupiter host star WASP-104th The ellipsoidal modulation is detected with high significance and in accordance with theoretical expectations, while Doppler radiation and reflection modulations are experimentally detected. We show that the visual geometric albedo at 95% confidence is lower than 0.03, making it one of the least reflective planets ever found. The light curve also shows a rotation modulation that implies a stellar rotation period near 23 or 46 days. In addition, we refine the system parameters and set strict upper limits for run-time and duration variations, star point coverage events, and additional planets in transit.