James Webb Space Telescope directly images its coldest exoplanet target yet

Using the James Webb Space Telescope (JWST), astronomers have investigated a new “super-Jupiter” planet that is one of the coldest worlds ever seen outside the solar system.

The extrasolar planet or “exoplanet” is located in the triple star system Epsilon Indi, which sits around 12 light-years away. The planet is designated Eps Ind Ab and has a mass around six times that of Jupiter; it orbits its red dwarf parent star at a distance similar to that between Neptune and the sun. This gives the world a surface temperature of around 32 degrees Fahrenheit (0 degrees Celsius) and means one orbit of the planet takes around 200 Earth years.

The JWST was able to image the exoplanet using the sensitive infrared capabilities of the Mid-Infrared Instrument (MIRI). The investigation of Eps Ind Ab could help astronomers better understand the evolution of gas giant planets and their systems.

This is the first time that the powerful space telescope has been able to image an exoplanet that wasn’t previously imaged from the ground. Eps Ind Ab is also the coldest exoplanet the JWST has been able to study thus far.

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“We were excited when we realized we had imaged this new planet,” Elisabeth Matthews, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany and lead author of a study on the find, said in a statement. “To our surprise, the bright spot that appeared in our MIRI images did not match the position we were expecting for the planet.”

Matthews added that previous studies had correctly identified a planet in this system, but had underestimated this super-Jupiter gas giant’s mass and orbital separation. The team was able to correct this with the help of the JWST.

The crew determined the planet has a highly elliptical, or “flattened,” orbit that, at its closest, brings it to a distance about 20 times the distance between Earth and the sun. At the furthest point in its orbit, Eps Ind Ab is 50 times further away from its parent star than the average distance between Earth and the sun.

The world’s parent star, a red dwarf named Eps Ind A, has two stellar companions. Both are brown dwarfs, or “failed stars,” named as such because they’re objects that form like stars but can’t gather enough mass to trigger nuclear fusion processes in their core that define what a “main sequence star” is.

Cold planets; hot topic

Thus far, scientists have only been able to detect a scattered few cold planets with wide orbits outside the solar system. And even those have been detected indirectly, through a technique of exoplanet detection called the “radial velocity method,” which measures the “wobble” an orbiting planet causes in the motion of its parent star.

Planets like this are tough to detect with other methods. Cold planets are far from their stars, and their orbits are unlikely to cause planets to cross, or “transit,” the face of their star (as seen from our position in the solar system). Such transits are typically important for exoplanet detection because they cause a blips in starlight headed for detectors in our vicinity. More specifically, a star’s light would appear to “dip” during a transit.

This means the transit method of exoplanet detection is mostly out of the question for these planets, but the radial-velocity method is also unreliable because only a small amount of wide orbits produces a detectable “tug” on a star.

The radial-velocity method had been used to study Eps Ind Ab, but because only a small slice of its orbit could be investigated, this led to the incorrect conclusion that the super-Jupiter took just 43 Earth years to complete an orbit of its star. Years of observation would be required to get a more precise picture of this planet’s orbit with this method.

Aware of these difficulties, Matthews and colleagues decided to take a different approach, attempting to directly image Eps Ind Ab.

Directly imaging an exoplanet is a tough proposition. Not only are the closest exoplanets many light-years away, but most astronomy cameras are also blinded by the bright lights of stars when trying to look at orbiting planets.

So, the team exploited MIRI’s “coronagraph,” a starlight-blocking shield that basically simulates an eclipse. MIRI was also the ideal instrument for this investigation because it “sees” the cosmos in thermal, or mid-infrared, light. This is the kind of light cold objects brightly emanate.

The researchers were aided by the fact that, at just 12 light-years away, Eps Ind Ab is relatively close to Earth. The smaller distance between the JWST and this target exoplanet meant the distance between Eps Ind Ab and its star appeared larger. This apparent wider separation means there is a better chance to mitigate the blinding effect of starlight.

“We discovered a signal in our data that did not match the expected exoplanet,” Matthews said, indicating that Eps Ind Ab appeared as a point of light in the MIRI image that was not in the predicted location. “But the planet still appeared to be a giant planet.”

To confirm this, the team still had to ensure what they saw wasn’t the result of background light coming from a more distant star. Thanks to re-examinations with the same region using the Very Large Telescope (VLT), the scientists found a faint object. This object happened to be at the right spot if the signal indeed belonged to the star Eps Ind A.

Matthews and colleagues also attempted to understand the atmosphere of Eps Ind Ab using the MIRI data. This revealed that the super-Jupiter appears packed with heavy elements, particularly carbon, which builds molecules such as methane, carbon dioxide and carbon monoxide, which are commonly found in gas-giant planets.

An alternative explanation for this is that the planet’s atmosphere is cloudy. All of this is to say that more research is needed to better understand Eps Ind Ab.

The team now intends to obtain spectra of light from Eps Ind Ab that could provide a detailed “fingerprint” of the super-Jupiter’s chemical composition and general climate.

“In the long run, we hope to also observe other nearby planetary systems to hunt for cold gas giants that may have escaped detection,” Matthews concluded. “Such a survey would serve as the basis for a better understanding of how gas planets form and evolve.”

The team’s research was published on Wednesday (24 July) in the journal Nature.