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Using the James Webb Space Telescope (JWST), astronomers have studied a new “super-Jupiter” planet, one of the coldest worlds ever observed outside the solar system.
The exoplanet, or “exoplanet,” is located in the Epsilon Indi triple star system, about 12 light-years away. Designated Eps Ind Ab, the planet has a mass about six times that of Jupiter and orbits its red dwarf parent star at a distance similar to that between Neptune and the sun. As a result, the world has a surface temperature of about 32 degrees Fahrenheit (0 degrees Celsius) and takes about 200 Earth years to orbit.
The JWST was able to image the exoplanet using the sensitive infrared capabilities of the Mid-Infrared Instrument (MIRI). The research from Eps Ind Ab can help astronomers better understand the evolution of gas giant planets and their systems.
This is the first time the powerful space telescope has been able to image an exoplanet that had not been imaged from the ground before. Eps Ind Ab is also the coldest exoplanet the JWST has been able to study so far.
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“We were excited when we realized that we had captured 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 expected for the planet.”
Matthews added that previous studies had correctly identified a planet in this system, but had underestimated the mass and orbital separation of this super-Jupiter gas giant. The team was able to correct this using the JWST.
The crew determined that the planet has a highly elliptical, or “flattened,” orbit that takes it to a distance of about 20 times the Earth-Sun distance at its closest point. At its farthest point in its orbit, Eps Ind Ab is 50 times farther from its parent star than the average Earth-Sun distance.
The world’s parent star, a red dwarf called Eps Ind A, has two stellar companions. Both are brown dwarfs, or “failed stars,” so called because they are objects that form as stars but cannot accumulate enough mass to trigger the nuclear fusion processes in their cores that define what a “main sequence” star is.
Cold planets; hot topic
So far, scientists have only been able to detect a few scattered cold planets with wide orbits outside the solar system. And even those have been detected indirectly, via a technique for detecting exoplanets called the “radial velocity method,” which measures the “wobble” an orbiting planet causes in the motion of its parent star.
Planets like these are difficult to detect using other methods. Cold planets are far from their stars, and their orbits are unlikely to cause the planets to cross, or “pass” across, 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 blip in the starlight traveling toward detectors in our vicinity. More specifically, the light from a star appears to “dip” during a transit.
This means that the transit method for detecting exoplanets is virtually impossible for these planets, but the radial velocity method is also unreliable, as only a small amount of wide orbits will produce a detectable ‘pull’ on a star.
The radial velocity method had been used to study Eps Ind Ab, but because only a small portion of its orbit could be examined, it led to the incorrect conclusion that the super-Jupiter took only 43 Earth years to complete one orbit around its star. Years of observation would be needed to obtain a more accurate picture of the planet’s orbit using this method.
Matthews and his colleagues were aware of these difficulties and decided to take a different approach: they attempted to image Eps Ind Ab directly.
Directly imaging an exoplanet is a difficult task. Not only are the nearest exoplanets many light-years away, but most astronomy cameras are blinded by the bright lights of stars when trying to look at orbiting planets.
So the team used MIRI’s “coronagraph,” a starlight-blocking shield that essentially simulates an eclipse. MIRI was also the ideal instrument for this research because it “sees” the cosmos in thermal, or mid-infrared, light. This is the kind of light that cold objects radiate brightly.
The researchers were helped by the fact that Eps Ind Ab, at just 12 light-years away, is relatively close to Earth. The closer proximity of the JWST to this exoplanet meant that the distance between Eps Ind Ab and its star appeared greater. This apparent greater distance means that there is a better chance of mitigating the blinding effect of starlight.
“We found a signal in our data that didn’t match the expected exoplanet,” Matthews said, indicating that Eps Ind Ab appeared as a bright spot in the MIRI image that wasn’t in the predicted location. “But the planet still appeared to be a giant planet.”
To confirm this, the team still had to make sure that what they were seeing wasn’t the result of background light coming from a more distant star. By re-examining the same region using the Very Large Telescope (VLT), the scientists found a faint object. This object just happened to be in the right place if the signal really did belong to the star Eps Ind A.
Matthews and colleagues also tried to understand Eps Ind Ab’s atmosphere using the MIRI data. This revealed that the super-Jupiter is packed with heavy elements, particularly carbon, which builds molecules such as methane, carbon dioxide and carbon monoxide, which are common in gas giant planets.
An alternative explanation for this is that the planet’s atmosphere is cloudy. All of this means that more research is needed to better understand Eps Ind Ab.
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The team now plans to obtain light spectra of Eps Ind Ab that could provide a detailed ‘fingerprint’ of the super-Jupiter’s chemical composition and overall climate.
“In the long term, we hope to observe other nearby planetary systems to hunt for cold gas giants that may have gone unnoticed,” Matthews concluded. “Such a study would serve as a foundation for a better understanding of how gas planets form and evolve.”
The team’s research was published Wednesday (July 24) in the journal Nature.