JWST has obtained a direct image of its first exoplanet, a temperate super-Jupiter just 12 light-years from Earth. It could be the oldest and coldest planet ever detected.
The planet orbits the star Epsilon Indi A (Eps Ind A), a K-type star It is about the same age as our Sun. Epsilon Indi is a triple star system, the other two members of which are brown dwarfs. The exoplanet is called Epsilon Indi Ab (Eps Ind Ab).
The detection of Eps Ind Ab is presented in an article published in Nature. Its title is “A temperate superJupiter imaged with JWST in the mid-infrared.The lead author is Elisabeth Matthews, a postdoctoral researcher in the Department of Planet and Star Formation at the Max Planck Institute for Astronomy in Germany.
This new detection is important for several reasons. The vast majority of the more than 5,000 exoplanets we have discovered were detected by the transit method. Others were detected with the radial velocity methodComparatively few have been photographed directly as Eps Ind Ab has been.
There were already hints that a massive planet was orbiting Eps Ind A. Previous work using the radial velocity method detected the telltale wobble induced in the star by a massive planet orbiting it. Now, JWST has confirmed the planet's presence.
“Our previous observations of this system have been more indirect measurements of the star, which allowed us to see in advance that there was likely a giant planet in this system tugging on the star,” said team member Caroline Morley from the University of Texas at Austin. “That’s why our team chose this system to observe with Webb first.”
Direct images of exoplanets are difficult to obtain. The blinding light from the star washes out the relatively faint light coming from the planet. Telescopes like JWST use coronagraphs to remove the starlight and allow planetary light to pass through. In this case, the space telescope imaged the exoplanet using its Coronary imaging with mid-infrared instrument (MIRI) ability.
JWST's direct imaging of Eps Ind Ab revealed some surprises compared to previous radial velocity measurements.
“While we expected to image a planet in this system because there were hints of its presence through radial velocity, the planet we found is not what we had predicted,” Matthews shared. “It is about twice as massive, slightly farther from its star, and has a different orbit than we expected. The cause of this discrepancy remains an open question.”
Eps Ind Ab has a mass about 6 times that of Jupiter and its semi-major axis measures about 28 AU. It has an inclination of about 103 degrees.
“The planet’s atmosphere also appears to be a bit different than the model predictions,” Matthews added. “So far, we only have a few photometric measurements of the atmosphere, which means it’s hard to draw conclusions, but the planet is fainter than expected at shorter wavelengths.”
Eps Ind Ab is more similar to Jupiter than any other exoplanet imaged so far, even though it is slightly warmer and several times more massive. Other exoplanets imaged tend to be hotter and still radiate the heat of their formation. Their heat makes them easier to see in infrared. As planets like this one age, they tend to shrink and cool. As they cool, it can become harder to image them directly.
As planets age and cool, the wavelength of their emissions changes, making them harder to see. Most other directly imaged planets are much younger than Eps Ind Ab (all less than 500 million years old), but JWST is particularly well suited to detecting older exoplanets.
“Cool planets are very faint, and most of their emission is in the mid-infrared,” Matthews explained. “Webb is ideal for imaging in the mid-infrared, which is extremely difficult to do from Earth. We also needed good spatial resolution to separate the planet and star in our images, and the large Webb mirror is extremely helpful in this regard.”
Many of the Jupiter-sized exoplanets we've discovered are hot Jupiters. These gas giants are easily found using the transit method because they orbit very close to their stars, making them hot. They're also often tidally locked, meaning their daysides can reach extreme temperatures. One hot Jupiter, KELT-9b, has a dayside temperature exceeding 7,800 degrees Fahrenheit (4,600 Kelvin), which is hotter than most stars.
But Eps Ind Ab is different. At about 35 degrees Fahrenheit (2 degrees Celsius), it is one of the coldest exoplanets ever directly detected. It is the coldest exoplanet ever directly imaged, and it is only 180 degrees Fahrenheit (100 degrees Celsius) warmer than the gas giants in our Solar System. It is more similar to planets in our system and gives astronomers the opportunity to study the atmospheres of Solar System analogs.
The planet's atmosphere doesn't exactly match our expectations. “The planet's atmosphere also appears to be a bit different than the model predictions. So far we only have a few photometric measurements of the atmosphere, which means it's hard to draw conclusions, but the planet is fainter than expected at shorter wavelengths,” Matthews said.
It could be weaker in those near-infrared wavelengths because the atmosphere is cloudy. Or it could be because it contains compounds like CH4.4 (methane), CO and CO2 that absorb shorter wavelengths of IR light.
The weakness of Eps Ind Ab at those wavelengths suggests a high carbon-to-oxygen ratio. A high C/O ratio is a significant indicator of how the planet formed and evolved. It suggests that the disk in which the planet formed was carbon-rich. It's a clue to where exactly the planet formed and whether it migrated.
The high carbon content also allows for more carbon-containing molecules, such as CH4CO2 and CO to form. Since CO2 and methane are greenhouse gases, the high C/O ratio affects the planet's climate.
High C/O ratios also affect cloud formation, which can increase a planet's albedo. A higher albedo reflects more sunlight away from the planet, which also affects climate.
Eps Ind Ab shows a high metallicity. A high metallicity indicates a larger mass and suggests a more efficient formation process, as the planet could have attracted more mass more quickly. It may also affect how the planet may have migrated through the disk.
Researchers are wondering whether other cold exoplanets have the same characteristics, but they first need to better delineate these features on Eps Ind Ab. This initial detection and imaging is just the beginning. Future spectroscopy and additional imaging will reveal more details about the planet.
The fact that Eps Ind Ab is in a cluster with two brown dwarfs is also an opportunity for more interesting observations. “The system also moves together with a widely separated brown dwarf binary system, making it a particularly valuable laboratory for comparative studies of substellar objects with a shared age and formation location,” the authors write in their paper. The line of demarcation between massive gas giants and brown dwarfs is not always clear, and astronomers are eager to learn more about how each type forms, especially in the same system.
This research also illustrates the effectiveness of using previous results from other telescopes to select targets for JWST. “Although the detected planet does not match the exoplanet properties that had been previously claimed, the long-term VR data provided a clear signal of the value of imaging this target,” the authors explain.
Fortunately, the exoplanet is an excellent candidate for further observations.
They conclude that “the bright flux and wide separation of Eps Ind Ab mean that the planet is ideally suited for spectroscopic characterization efforts, allowing more precise constraining of metallicity and carbon-to-oxygen ratio.”
