Has this hot planet lost – and regained – an atmosphere?


Laura Kreidberg, who heads research on exoplanet atmospheres at the Max Planck Institute, would like to see independent analysis of the data before jumping to conclusions. “There are a lot of small decisions in data processing that can produce unexpected jerks and tremors,” says Kreidberg. “I would like to see the spectrum reproduced by another team using independent methods to see if they get the same.”

In fact, this process is already underway. Last week, another research team led by Lorenzo Mugnai, an astrophysicist at the Sapienza University in Rome, published another paper which independently analyzes the same Hubble data on GJ 1132 b. But when Mugnai’s team analyzed the data, they found that the planet’s spectrum was relatively flat – in other words, there was no detectable atmosphere. “It is very difficult to be sure of the cause of the differences, because it is a very difficult analysis,” Mugnai says. “We know the devil is in the details.”

The two teams meet regularly to determine what led to such a dramatic discrepancy in their results, but Mugnai and Swain both believe the problem could lie in how they explain the variation in sunlight as the planet turns. moves in front of its star, a parameter called limb darkening. “A star is not uniform in brightness from center to edge,” Swain says. “When the planet is close to one edge or another, it seems to block less light, because part of the star it covers is on average darker than the rest of the star.”

To correct for this effect, researchers must process their data with a model that can take into account the gradation and brightening of the star. The two teams used the same model, but with different coefficients. They are now planning to exchange methods to see if they can replicate the other team’s results.

Even so, Darius Modirrousta-Galian, the co-author of Mugnai’s article, thinks it is highly unlikely that GJ 1132b could have retained enough hydrogen to produce a second atmosphere because it is so close to its star. host. Exoplanet researchers still do not know how much stellar radiation can influence the formation of atmospheres. “The approach we take is that the stellar irradiation is actually so strong and the winds on the planet have supersonic speeds and extreme particle speeds, that the atmosphere is dissipating,” he says.

Modirrousta-Galian says the amount of hydrogen in the primordial envelope that would be needed to overcome this loss and create a second atmosphere would be several times the mass of the planet. “We have no problem in our model that the planet could have been born with an atmosphere of hydrogen,” he says. “The conclusion we came to is that we just don’t have one now.”

Yet more research – and ideally new observations by the James Webb Space Telescope, launch scheduled for October 31—Is necessary to verify or further complicate the results of either team. If GJ 1132b turns out to have a hydrogen atmosphere, it could open up new avenues of exploration for planetary scientists. On the one hand, these atmospheres would be much easier to analyze than those of small planets with denser envelopes made up of heavier elements. Hydrogen’s low molecular weight contributes to a larger, more puffy atmosphere for light to shine through. And this allows for a stronger spectrographic signature that is easier to read from Earth.

The two teams are pushing the limits of what is possible with the Hubble Space Telescope, which launched in 2000, two years before astronomers discovered the first known exoplanet. At 1.16 times the size of Earth, GJ 1132b is the smallest planet that has ever had a published transmission spectrum, Swain notes. “I think what’s exciting here is to better understand the details that really matter for the study of small planets,” he says.


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