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. Astronomers initially thought the planet could host a deep ocean of liquid water, but the latest analysis suggests it could be fundamentally different from anything seen before.
“In reality, everything is in a soft, molten state,” said Dr. Harrison Nicholls, an astrophysicist at the University of Oxford. “It’s like molasses. The core of this planet is probably molten, too.”
Surface temperatures would reach a scorching 1,900°C (3,500°F), large waves would likely roll over the magma ocean caused by tidal forces from neighboring planets, and there would be a widespread stench of rotten eggs thanks to an atmosphere rich in hydrogen sulfide. The general conditions are not considered favorable for host life.
“If there were aliens that could live in lava, that would be amazing, but I don’t think it’s likely to be habitable,” Nicholls said. “It’s nice to revel in the otherworldliness of the planet itself.”
The planets beyond our solar system are too distant to photograph or reach with robotic spacecraft, and until recently astronomers could only make rough estimates of the size, density and temperature of these distant worlds by following their silhouettes as they passed in front of their host star. The James Webb Space Telescope, however, is powerful enough to measure starlight filtered through the planet’s atmosphere and give a reading of what gases are present.
Previously, such observations revealed that L98-59d has a sulfur-rich atmosphere that seemed to contradict it being a rocky or aquatic world, the two conventional categories a planet of its size would normally fall into. Neither of them would be able to maintain a sulfur atmosphere for the almost 5 billion years that the planet has existed.
Using advanced computer simulations, the latest research reconstructed the planet’s history from shortly after its birth 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,” Nicholls said. “The magma ocean efficiently stores 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 that astronomers may need to be more cautious when designating exoplanets as potentially habitable.
“Some planets in the so-called habitable zone might not be habitable at all, they might be these molten planets,” Nicholls said. “While this molten planet is unlikely to support life, it reflects the wide diversity of worlds that exist beyond the solar system. So we can ask, what other types of planets are waiting to be discovered?”
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 provides a plausible explanation.
“We’re talking about it possibly being an exoplanet that looks like Io, Jupiter’s moon, with a lot of volcanoes caused by tidal heating,” he said. “This work suggests it could be even more extreme.”
The findings are published in the journal Nature Astronomy.
