Researchers have used the James Webb Space Telescope to measure the temperature of the innermost planet in the TRAPPIST-1 system, shedding more light on the ability of planets like these to support life.
TRAPPIST-1 is an ultracool red dwarf star (or M dwarf) barely larger than Jupiter that sits about 40 light-years away and is orbited by seven Earth-sized exoplanets. It’s up to twice as old as our own solar system, making it perfect for studying the formation and evolution of terrestrial planets.
The innermost of the known rocky exoplanets in the system, TRAPPIST-1 b is slightly larger than Earth but has the same density. Although it’s not within the system’s “habitable zone” like planets e, f, and g are, it can still provide important information about the other planets in the TRAPPIST-1 system and those of other M-dwarf systems.
An international team of researchers has used NASA’s James Webb Space Telescope (JWST) to take a closer look at TRAPPIST-1 b, to determine whether the planet has an atmosphere, with a view to applying the information obtained to the habitability of other exoplanets.
“If we want to understand habitability around M stars, the TRAPPIST-1 system is a great laboratory,” said Elsa Ducrot, co-author of the study. “These are the best targets we have for looking at the atmosphere of rocky planets.”
One of the factors that determine a planet’s habitability is the existence of an atmosphere. Previous observations of TRAPPIST-1 b using the Hubble and Spitzer space telescopes found no evidence of a puffy atmosphere – one that is significantly larger than expected – but couldn’t rule out a dense one. Seeking an answer to the question about atmosphere, the research team measured the planet’s temperature.
“This planet is tidally locked, with one side facing the star at all times and the other in permanent darkness,” said Pierre-Olivier Lagage, one of the study’s authors. “If it has an atmosphere to circulate and redistribute the heat, the dayside will be cooler than if there is no atmosphere.”
TRAPPIST-1 b doesn’t give off its own visible light because it’s not hot enough, but it does emit an infrared glow. Using the JWST’s mid-infrared instrument (MIRI), researchers were able to calculate the amount of heat, given off as infrared light, produced by the planet.
“These observations really take advantage of Webb’s mid-infrared capability,” said Thomas Greene, lead author of the study. “No previous telescopes have had the sensitivity to measure such dim mid-infrared light.”
They found that TRAPPIST-1 b’s dayside has a temperature of around 500 Kelvin (about 450 °F or 227 °C), suggesting the planet has little or no atmosphere.
This is the first time that researchers have been able to detect any form of light emitted by an exoplanet as small and cool as the rocky planets in our own solar system. For the research team, it’s an important step toward discovering whether planets in the TRAPPIST-1 system and those like them, can sustain life-supporting atmospheres.
Another major milestone was achieved by the researchers: the detection of a secondary eclipse. In a secondary eclipse, the planet crosses behind the star, blocking the light from the planet. The MIRI measured the change in brightness from the system as TRAPPIST-1 b moved behind the star.
Data from five separate secondary eclipses were analyzed and compared to computer models showing what the temperature should be in different scenarios.
“The results are almost perfectly consistent with a blackbody made of bare rock and no atmosphere to circulate the heat,” Ducrot said. “We also didn’t see any signs of light being absorbed by carbon dioxide, which would be apparent in these measurements.”
A blackbody is an object that absorbs all the radiation falling on it. It does not reflect or transmit any light nor allow light to pass through it and out the other side. The energy a blackbody absorbs heats it up, causing it to emit its own radiation. Temperature is the only parameter that determines how much light the blackbody gives off.
Now that they have successfully captured a secondary eclipse, the researchers are using the JWST’s MIRI to obtain additional observations of the phenomenon, hoping to capture a full phase curve showing the change in brightness over the planet’s entire orbit. This would enable researchers to compare temperature changes between dayside and nightside and confirm the existence of an atmosphere.
“This is the first time we can detect the emission from a rocky, temperate planet," Lagage said. "It’s a really important step in the story of discovering exoplanets."
The study was published in the journal Nature.
