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Astronomers have identified a planet composed of molten lava, suggesting the existence of an entirely new category of liquid planet.
The distant world, known as L98-59d, is about 1.6 times the size of Earth and orbits a small red star 35 light years away. Initially, astronomers thought the planet might harbor a deep ocean of liquid water, but the latest analysis suggests it could be fundamentally different from anything seen before.
“The whole thing really is in a mushy, molten state,” said Dr. Harrison Nicholls, an astrophysicist at the University of Oxford. “It’s like molasses. It’s likely that this planet’s core would also be molten.”
Surface temperatures would reach a blazing 1,900°C (3,500°F). Large waves are likely to roll over the magma ocean due to tidal forces from neighboring planets, and the atmosphere, rich in hydrogen sulfide, would emit a pervasive stench of rotten eggs. These conditions are not considered favorable for hosting life.
“If there are aliens out there that could live in lava, that would be amazing, but I don’t think it’s likely that it’s habitable,” said Nicholls. “It’s nice to revel in the alienness of the planet itself.”
Planets beyond our solar system are too distant to photograph or reach with robotic spacecraft. Until recently, astronomers could only make crude estimates of the size, density, and temperature of these distant worlds by tracking their silhouettes as they passed in front of their host star. However, the James Webb Space Telescope is powerful enough to measure starlight filtered through the planet’s atmosphere, providing a readout of which gases are present.
Previous observations revealed that L98-59d has a sulfur-rich atmosphere that appeared inconsistent with it being either a rocky or water world — the two conventional categories for a planet of its size. Neither type would be capable of maintaining a sulfur atmosphere for the nearly 5 billion years the planet has existed.
Using advanced computer simulations, the latest research reconstructed the planet’s history from shortly after its formation to the present day. This suggested that L98-59d has a global magma ocean extending thousands of kilometers beneath its surface — and possibly a molten core.
“You can only really explain this planet if it has this deep magma ocean inside of it,” said Nicholls. “The magma ocean efficiently stores the gases and keeps them protected from physical processes that would otherwise remove them.”
The findings, which imply that molten planets may be quite common, suggest astronomers may need to be more cautious about designating exoplanets as potentially habitable.
“Some planets in the so-called habitable zone might not be very habitable at all; they might be these molten planets,” said Nicholls. “While this molten planet is unlikely to support life, it reflects the wide diversity of worlds that exist beyond the solar system. We may then ask, what other types of planets are waiting to be uncovered?”
Dr. Jo Barstow, a planetary scientist at the Open University who was involved in observations of L98-59d with the James Webb telescope, said the latest work provided a plausible explanation.
“We talked about it possibly being an exoplanet that resembles Jupiter’s moon Io, with lots of volcanoes caused by tidal heating,” she said. “This work suggests it could be even more extreme.”
The findings are published in the journal Nature Astronomy.
