Seven hours: that's how long a year lasts on TOI-4552b. It whips around its star faster than some moons orbit Earth. The star is a dim red dwarf, four times smaller than the Sun, but the planet is so close that it receives hundreds of times more heat than we do.
A team of astronomers confirmed the discovery made by the TESS space telescope using a dozen ground-based telescopes. The planet is real, not a measurement error: its shadow against the star was seen by several observatories at different points on Earth. The NIRPS and HARPS spectrographs measured the star's slight wobble caused by the planet's gravity, yielding a mass of 1.83 Earth masses. Its radius is slightly larger than Earth's—1.11. These two numbers reveal its composition: TOI-4552b has more iron inside than we do. The planet's core takes up about half its volume, compared to about a third for Earth.
The planet is interesting not only for its record-breaking short year. Its star is surprisingly calm: it has almost no spots or flares, which usually hinder observations of a planet's atmosphere. And the atmosphere itself, if it exists, is intensely heated and thus glows brightly in infrared. The James Webb Space Telescope can split this light into colors and reveal what the air of this iron world is made of. If we're lucky, we'll see the atmosphere of a rocky planet around a red dwarf for the first time.
Seven hours, and dawn again. That's the rhythm of life on TOI-4552b, if you can call it life.
TOI-4552b's density is 7.74 grams per cubic centimeter — almost that of pure iron. A dice-sized cube of this planet would weigh as much as a good hammer.
In the series 'The Expanse', colonists live on asteroids with short day cycles, but a planet with a seven-hour year is more like Arthur C. Clarke's 'Rendezvous with Rama', where time flows very differently.