Astronomers have discovered a strange giant planet that seems to break the usual rules of how planets form.
Known as TOI-5205 b, this world has been nicknamed a “forbidden planet” because it orbits a very small, cool star—something scientists once thought was unlikely for such a large planet.
TOI-5205 b is about the size of Jupiter, but its host star is tiny in comparison, only about four times the size of Jupiter and roughly 40% the mass of our sun.
This unusual pairing has already puzzled scientists, but new observations have revealed something even more surprising about the planet’s atmosphere.
Using the powerful James Webb Space Telescope, an international research team led by scientists from NASA Goddard Space Flight Center and Carnegie Science studied the planet as it passed in front of its star.
During this event, called a transit, the planet blocks about 6% of the star’s light. By analyzing how the starlight changes as it passes through the planet’s atmosphere, scientists can identify what gases are present.
The team detected methane and hydrogen sulfide in the atmosphere, giving clues about its chemical makeup.
But what really stood out was the planet’s “metallicity”—a term astronomers use to describe the amount of elements heavier than hydrogen and helium. Surprisingly, TOI-5205 b’s atmosphere contains fewer heavy elements than both its host star and even Jupiter in our own solar system.
This is unusual because planets are generally expected to have equal or higher metallicity than their stars, since they form from the same cloud of gas and dust.
According to Dr. Anjali Piette from the University of Birmingham, this makes TOI-5205 b very different from other giant planets studied so far and challenges current theories of planet formation.
To understand this better, the researchers used computer models to study the planet’s interior. These models suggest that while the planet as a whole is rich in heavy elements—about 100 times more than its atmosphere—those elements may have sunk deep inside the planet over time.
This would leave the outer atmosphere relatively “clean,” meaning it contains mostly lighter gases.
Another interesting finding is that the planet’s atmosphere appears to be rich in carbon but low in oxygen. This unusual balance could provide further clues about how the planet formed and evolved.
The study also faced a technical challenge. The host star is covered in starspots—dark, cooler regions similar to sunspots—which can interfere with the data by altering the observed light. The researchers developed methods to correct for these effects, improving the accuracy of their results.
This work is part of the GEMS Survey, a project focused on studying giant planets around small stars. Future observations will help confirm these findings and improve our understanding of how such unusual planets form.
Source: KSR.
