How different are day and night on TRAPPIST-1 b?
TRAPPIST-1 b is the innermost planet, orbiting its star about every 1.51 days. It is expected to keep one side facing the star, so without an atmosphere the dayside stays hot while the nightside receives very little heat. A thick atmosphere would use winds to carry heat into the night and reduce that difference. Webb measured the planet’s infrared light throughout one orbit and found a dayside brightness temperature of about 490 K, or 217°C, but did not clearly detect heat from the nightside. An atmosphere with surface pressure greater than about 1 bar—similar to pressure at Earth’s surface—did not fit the result well.
However, this does not confirm that there is not a single molecule of gas in TRAPPIST-1 b. An atmosphere that is either very thin or only traces of it in current observations may remain. The explanation that best fits the current data is that the dark, rocky surface with little atmosphere emits daytime heat directly into space.
- About 1.5109 days Time to circle the central star
- Approximately 1.72 million km Average distance between the planet and its star
- Approximately 1.37 times that of Earth Planetary mass determined by change in transit time
- Approximately 1.12 times that of Earth Planetary radius calculated from starlight occlusion ratio
How do we measure a planet’s dayside temperature?
When the planet is beside the star in our view, the telescope receives both starlight and infrared light from the planet’s dayside. Once the planet disappears completely behind the star, only the starlight remains. Measuring the difference reveals the heat from the dayside even though the telescope cannot resolve the two objects in an image. This is called a secondary-eclipse observation.
In 2023, Webb's MIRI instrument observed the planet hiding in infrared light with a wavelength of 15 μm five times. When the planet hid, the total light decreased by about 0.08%, and the brightness temperature was calculated to be about 500K, or about 227 degrees Celsius. Brightness temperature is the value of brightness measured under specific light converted to temperature. It is not a direct measurement of the average temperature of the entire planet or the temperature everywhere on the surface.
What can one kind of infrared light tell us?
The result of the day side appearing bright in 15μm infrared light fits well with the explanation that there is no atmosphere and the dark surface absorbs a lot of starlight. If there was a thick carbon dioxide atmosphere, it would absorb this infrared radiation and move away the daytime heat, making the daytime side appear darker. This is why the 2023 results first strongly showed the possibility that there is no thick atmosphere.
However, only one type of infrared light can reveal only a portion of the planet's heat. In 2024, five observations with a wavelength of 12.8 μm were added. Looking at the two types of infrared light together, the rocky surface with no atmosphere and a lot of iron and magnesium matched the data well. However, a thick carbon dioxide atmosphere with a high haze layer could produce the same brightness under some conditions. It was not possible to determine which explanation was correct based on the two types of infrared rays alone.
The moment when the planet hides behind the star reveals only its dayside heat. To learn whether an atmosphere carries heat into the night, Webb must follow the planet throughout a full orbit.
How does the heat change over a full orbit?
As the planet orbits the star, the side facing Earth continues to change. Webb kept track of how bright it was when the day side was visible, how dark it was when the night side was visible, and when it was brightest. This change in brightness is called a thermal phase curve. If the atmosphere transfers heat, infrared rays come out at night, and the hottest point can move sideways from the point where the star is directly visible.
In the results announced in 2026, the daytime brightness temperature was 490 ± 17 K, approximately 217 ± 17 degrees Celsius. The heat on the night side was not clearly visible, and the point of greatest brightness did not shift noticeably. Rather than a thick atmosphere that widely transfers daytime heat into the night, a better explanation is that it is a rocky planet whose daytime surface radiates heat directly into space. In an atmosphere where the surface pressure exceeds about 1 bar, the possibility is greatly reduced.
- 2023 The daytime heat and brightness temperature of about 500K were obtained through five observations with a wavelength of 15μm.
- 2024 Five observations with a wavelength of 12.8 μm were added, but both rock surfaces and some thick atmosphere were possible.
- Full-orbit observation Webb continuously recorded the change in infrared brightness from day to night.
- Analysis for 2026 The hot day and indistinct nighttime heat greatly reduced the likelihood of a thick atmosphere.
Does TRAPPIST-1 b have an atmosphere?
What is most inconsistent with current observations is the thick atmosphere that transfers daytime heat into the night. It cannot be said that there are no very thin atmospheres or gases with weak infrared traces. How thin the atmosphere must be also depends on the type of gas, clouds, haze, and how much the surface reflects starlight.
The starlight when the planet passes in front of the star also shows that it does not have a thick atmosphere rich in hydrogen. However, the star spots and bright areas changed the starlight significantly, making it impossible to tell if there was a thin atmosphere of heavier gases. Observations during one revolution, when passing in front of a star and when hiding behind a star, have different weaknesses. The more methods that show the same conclusion, the more certain the judgment about the atmosphere becomes.
How can we learn the surface’s material and age?
The average density calculated based on mass and size is consistent with the possibility that b is a planet mainly composed of rock. The infrared brightness of both types matched well with calculations that included rock surfaces rich in iron and magnesium, such as olivine and pyroxene. However, this is only the result of comparing available materials. Webb did not analyze rock fragments or discover surface minerals directly.
If the surface is truly fresh, there may be processes by which volcanic activity or space weathering renew the surface, but the process and timing cannot be determined based on current data alone. Other less reflective rocks or surfaces that have experienced space weathering should also be considered. The lava, cracks, and colors seen in the planet's illustrations on the page are not taken from actual terrain.
About 490K, about 217 degrees Celsius, is the daytime brightness temperature obtained by infrared rays with a wavelength of 15μm. It is not the same as the average temperature of the entire planet, the highest surface temperature, or the temperature at night. Current observations greatly reduce the likelihood of a thick atmosphere, but do not single out all thin atmospheres and surface materials.
How does observing b help us study the other planets?
TRAPPIST-1 b is much closer to the star than the habitable zone and receives about four times more energy than Earth. Rather than a planet to first investigate the possibility of life, it is an object to find out what kind of infrared radiation a rocky planet with almost no atmosphere near a red dwarf star emits.
The data obtained when b passes in front of the star is also used as comparative data to find changes in starlight when observing an outer planet such as e. The more convincing the explanation that b has little atmosphere, the easier it will be to distinguish changes in the star itself from small traces left by the atmospheres of other planets. Conclusions about the atmosphere of b do not come from a single temperature measurement. This is the result of sequentially comparing two different types of infrared rays and the change in brightness of an entire cycle.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 b
- NASA Science — Webb Measures the Temperature of TRAPPIST-1 b
- Nature — Thermal Emission from TRAPPIST-1 b
- Nature Astronomy — Combined 12.8 and 15μm Eclipse Analysis
- Nature Astronomy — Thermal Phase Curves of TRAPPIST-1 b and c
- The Planetary Science Journal — TRAPPIST-1 Masses and Radii
- The Astrophysical Journal Letters — TRAPPIST-1 b Transmission and Stellar Contamination