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Astronomers have used the James Webb Space Telescope (JWST) to detect 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, about 12 light-years away. The planet is called Eps Ind Ab and has a mass about six times that of Jupiter; it orbits its red dwarf parent star at a distance similar to the distance between Neptune and the Sun. This gives the Earth a surface temperature of about 32 degrees Fahrenheit (0 degrees Celsius), and means that one orbit of the planet takes about 200 Earth years.
JWST was able to image the exoplanet using the sensitive infrared capabilities of its Mid-Infrared Instrument (MIRI). Eps Ind Ab’s research could help astronomers better understand the evolution of gas giant planets and their systems.
This is the first time the powerful space telescope has captured an image of an exoplanet that has not been previously imaged from the ground. Eps Ind Ab is also the coldest exoplanet JWST has been able to study so far.
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“We were excited when we realized we had imaged this new planet,” says Elisabeth Matthews, a researcher at the Max Planck Institute for Astronomy in Heidelberg, Germany, and lead author of a study on the find. “To our surprise, the bright spot visible in our MIRI images did not match the location we expected for the planet.”
Matthews added that previous studies had correctly identified a planet in this system but underestimated the mass and orbital separation of this super-Jupiter gas giant. The team was able to correct this with the help of JWST.
The crew determined that the planet has a highly elliptical, or “flattened”, orbit, and at its closest point it is about 20 times the distance between Earth and the Sun. At its farthest point in its orbit, Eps Ind Ab is 50 times farther from its parent star than the average distance between Earth and the Sun.
Earth’s parent star, a red dwarf named Eps Ind A, has two stellar companions. Both are brown dwarfs, or “failed stars,” objects that formed as stars but failed to gather enough mass in their cores to trigger the nuclear fusion processes that define what a “main sequence star” is.
Cold planets; hot topic
So far, scientists have only detected a few scattered, cold planets with wide orbits outside the solar system. And even those have been detected indirectly, using an exoplanet-detection technique called the radial velocity method, which measures the “wobbling” caused by an orbiting planet in the motion of its host star.
These types of planets are difficult to detect by other methods. Cool planets are far from their stars, and their orbits are unlikely to cause the planets to cross or “cross” the face of their stars (as seen from our position in the solar system). Such transits are often important for exoplanet detection because they cause a fluctuation in the starlight coming toward nearby detectors. More specifically, the light from a star appears to “fall” during a transit.
This means that the transit method for exoplanet detection is largely out of the question for these planets, but the radial velocity method is also unreliable because only a small number of wide orbits produce a detectable “tug of war” on a star.
The radial velocity method was used to study Eps Ind Ab, but because only a small slice of its orbit could be studied, this led to the incorrect conclusion that the super-Jupiter takes only 43 Earth years to complete its orbit around its star. It would take years of observations to obtain a more accurate picture of this planet’s orbit using this method.
Aware of these challenges, Matthews and colleagues decided to take a different approach and attempted to image Eps Ind Ab directly.
Imaging an exoplanet directly is a challenge. Not only are the closest exoplanets light-years away, but most astronomy cameras are blinded by the bright light of stars as they try to peer at the orbiting planets.
So, the team used MIRI’s “coronagraph,” basically a starlight-blocking shield that simulates an eclipse. MIRI was also an ideal tool for this research because it “sees” the cosmos in thermal, or mid-infrared, light. This is the type of light that cold objects emit brightly.
The researchers were helped by Eps Ind Ab’s relative proximity to Earth, at just 12 light-years away. The smaller distance between JWST and this target exoplanet meant that the distance between Eps Ind Ab and its star appeared larger. This apparent wider separation meant there was a better chance of reducing the blinding effect of the starlight.
“We discovered a signal in our data that did not match the expected exoplanet,” Matthews said, noting 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 had to make sure that what they were seeing wasn’t the result of background light from a more distant star. Thanks to a re-examination of the same area using the Very Large Telescope (VLT), the scientists found a faint object. If the signal really belongs to the star Eps Ind A, then this object is in the right spot.
Matthews and his colleagues also used MIRI data to understand Eps Ind Ab’s atmosphere, which revealed that the super-Jupiter appears to be full of heavy elements, especially carbon, which makes up molecules such as methane, carbon dioxide and carbon monoxide, which are common on 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.
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The team now aims to obtain light spectra from Eps Ind Ab, which 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 escaped detection,” Matthews concluded. “Such research will lay the foundation for a better understanding of how gas planets form and evolve.”
The team’s research was published in the journal Nature on Wednesday (July 24).