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Astronomers have photographed a planet six times more massive than Jupiter orbiting one of the closest stars to the Sun. It's the first extrasolar planet, which was discovered by direct imaging with the James Webb Space Telescope.
“This is a cold planet,” says astronomer Elisabeth Matthews of the Max Planck Institute for Astronomy in Heidelberg, Germany. The results were announced on July 24thNature 1published.
“If it is real, the planet is by far the oldest and coldest ever imaged,” says Markus Janson, an astronomer at Stockholm University.
Researchers typically discover exoplanets by tracking how they periodically cross the line of sight to Earth, temporarily dim the light of their host stars, or because their gravitational effects cause a measurable wobble in the star itself. So far only a few dozen exoplanets have been identified shown directly, typically because they are hot and bright enough to be visible despite the brightness of their stars.
Using wobble techniques, astronomers had already seen evidence that a massive object could be moving around the sun-like star ε Indi A, located just 3.6 parsecs (12 light-years) from Earth in the distant constellation Indus.
To search for a planet, Matthews and her colleagues aligned the telescope so that the star was exactly in the center of their field of view. They then used the “coronograph” feature built into one of the Webb’s cameras. The instrument can detect photons at slightly different times or phases in each of the four quadrants of its image frame. In this way, when the sensor data were combined, photons from ε Indi A itself - some of which deviate from the center - were mostly neutralized, removing the glare that would have drowned any other signal around the star.
The resulting image revealed a planet six times the mass of Jupiter, meaning that, like Jupiter, it must be a "gas giant" made up mostly of hydrogen gas, Matthews says. The planet, called ε Indi Ab, is about 15 times further from its host star than Earth is from the Sun, and its temperature is just above 0°C.
Janson warns that “the silver bullet to prove that it is definitely a planet” – a later image showing that the point of light has moved – does not yet exist. But, he adds, "the study is of great, great importance because it represents a step toward the ability to image planets in mature systems." Previous direct imaging efforts, he says, have been limited to observing young star systems, while ε Indi A is almost as old as the Sun.”
Matthews says her team plans follow-up observations to measure the planet's light spectrum. This could also reveal some of the components of its atmosphere, providing clues as to where and how such a large planet might have formed in the ancient nebula in which ε Indi A formed.
The star forms a triple system with two "brown dwarfs" - objects that never grew large enough to trigger hydrogen fusion in their cores - that orbit it more than 1,000 times farther from super Jupiter. “It's a really unusual type of system that we're really lucky to have in our backyard,” Matthews says.