Imagine a world so hot that its surface is an ocean of magma, yet enveloped in a dense and mysterious atmosphere. This is the case of TOI-561 b, a a super-Earth outside our solar system that challenges everything we thought we knew about rocky planets.
The planet TOI-561 b is enveloped in a thick atmosphere that covers a global ocean of magma. This discovery represents the strongest evidence yet of an atmosphere on a rocky exoplanet.
The discovery, led by scientists from the University of Birmingham (UK), was made possible thanks to the James Webb Space Telescope (JWST), operated collaboratively by NASA, the European Space Agency (ESA) and the Canadian CSA.
According to the researchers, this atmosphere could explain the planet’s unusually low density and, at the same time, shatters the belief that small planets close to their stars cannot maintain an atmosphere. A direct window into worlds that defy all known rules.
A planet with very low density
With a radius 1.4 times that of Earth and an orbital period of less than 11 hours, TOI-561 b belongs to an extremely rare class of ultra-short-period exoplanets, which orbit so close to their stars that they defy all expectations of what a rocky planet should be.
Although its star is only slightly smaller and cooler than the Sun, TOI-561 b orbits only 1.6 million kilometers awayexposing itself to dayside temperatures that far exceed the melting of the rock, transforming its surface into an ocean of incandescent magma.
The planet’s low density has baffled scientists. One possibility is that it has a relatively small iron core and a less dense rocky mantle than Earth, a design that gives it unusual properties. “What really sets this planet apart is its abnormally low density.. “It is less dense than we would expect if its composition was similar to that of Earth,” explains Johanna Teske, lead author of the study.
Additionally, TOI-561 b is unique among ultrashort period planets because it orbits a very old iron-poor star, twice as old as our Sun, in a region of the Milky Way called the thick disk. This suggests that the planet formed in a radically different chemical environment than our solar system and could be an example of what planets looked like during the first billion years of the universe.
A surprising atmosphere
Despite its scorching proximity to the star, TOI-561 b seems enveloped in a thick atmospherewhich might explain why it looks bigger than it really is. Small planets exposed to intense radiation for billions of years normally lose their atmosphere, but Webb detected something different.
With the NIRSpec Near Infrared Spectrographthe researchers measured the temperature on the dayside of the planet. If it were bare rock, the temperature would reach around 2,700ºC, but Webb recorded 1,800ºC, extremely hot, but significantly colder than expected. This suggests the presence of an atmosphere that transports heat and regulates temperature.
Several scenarios could explain this phenomenon: a magma ocean that a little heat circulates, or that a thin layer of rock vaporalthough none alone can completely justify the results. “We believe there is a balance between the magma ocean and the atmosphere. As gases escape from the planet, the ocean absorbs them again. “This planet must be much richer in volatile substances than Earth, really like a ball of wet lava,” describes Tim Lichtenberg, co-author of the study.
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