Surprise Planet Around a Young Star express
Astronomers caught the young planet AU Mic b in the act of hiding behind its star. The Spitzer telescope tracked its infrared glow: as the planet slipped behind, the system's total light dimmed slightly. The dip was just 171 parts per million, like a single drop of ink in ten buckets of water. But that was enough to measure the dayside temperature of this distant world.
The planet turns out to be scorching at 1031 Kelvin (about 758°C). For comparison, if it were heated only by its star's radiation, it would be around 606 Kelvin. That's the difference between a hot cup of tea and a sizzling skillet. The planet is clearly getting extra heat, but where from is a puzzle.
The scientists tested several explanations: maybe heat isn't distributed well across the planet, or it's contracting under its own gravity and warming up, or the star's pulsations are to blame, or even instrument errors. None fit. So we'll have to wait for new observations from the James Webb Space Telescope, which could peer into the planet's atmosphere and maybe find the source of the mysterious heat.
Original abstract
as written by the authors arXiv:2602.05168
We present the observation of a secondary eclipse of the young hot Neptune, AU Mic b, in the infrared using the Spitzer Space Telescope. Using a primary transit from Spitzer to constrain the system parameters, we tentatively detect an eclipse centered at $$BJD=2458740.848893^{+0.00010}_{-0.000099}$$ with an observed depth of $$171\pm{29}$$ ppm given an uninformed prior. This corresponds to a dayside brightness temperature of $$T=1031\pm{58}$$ K, which exceeds the calculated equilibrium temperature of $$606\pm{19}$$ K. We explore some possible explanations for these results, including inefficient heat redistribution, gravitational contraction, stellar pulsations, instrument systematics and choice of eclipse depth prior, but find none of these to be likely explanations for the observed eclipse parameters. We also explore the impact of correlated noise in the systematic trends, and we find that splitting the systematics into low-pass (smoothing) and high-pass trends is required to reach an optimal minimization of the low-frequency systematics in the resulting detrended light curve. Future observations with JWST are needed to confirm our eclipse detection with Spitzer.
The dayside temperature of AU Mic b is higher than that of some stars — red dwarfs that merely smolder in space.
In the novel "The Three-Body Problem," a planet in a three-sun system experiences chaotic temperature swings — AU Mic b also throws surprises, but for now it stays steadily hot.