What kind of atmosphere could remain on TRAPPIST-1 d?
TRAPPIST-1 d is a small rocky planet, with about 39% of Earth’s mass and 79% of its size. Webb analyzed several kinds of starlight during two transits, but found no clear marks from gases such as water, methane or carbon dioxide. The result does not fit a thick hydrogen-rich atmosphere or several clear atmospheres whose gases should have made large marks. Yet an almost airless surface, a very thin atmosphere, or an atmosphere whose lower gases are hidden by high clouds or haze all remain possible.
- About 4.0496 days Time required to orbit the star
- About 3.34 million km Average distance between the planet and star
- About 0.39 Earth masses Mass derived from changes in transit times
- About 0.79 Earth radii Radius derived from the fraction of starlight blocked
How do we search for an atmosphere in the starlight a planet blocks?
When a planet passes in front of its star, the planet itself blocks starlight and some light passes through the atmosphere around its edge. If a gas in the atmosphere absorbs a certain kind of light, the atmosphere becomes opaque at that wavelength and the planet appears slightly larger. Webb looks for atmospheric gases by comparing how much light is blocked at each wavelength. This is called a transmission spectrum.
The marks from gases become larger when the atmosphere is hotter, the planet’s gravity is weaker and the molecules are lighter. Planet d has weaker gravity than Earth, so a thick atmosphere of a light gas such as hydrogen should make its apparent size change clearly with wavelength. No such change appeared, making a thick hydrogen-dominated atmosphere very unlikely.
Which gases did two transit observations look for?
Webb’s NIRSpec instrument recorded light from the red edge of the visible range through the near infrared during two transits. Researchers also calculated how the spots and bright regions on TRAPPIST-1 changed the starlight. They then compared the data with possibilities ranging from no atmosphere to atmospheres with different pressures, gases and cloud heights.
No clear marks appeared from water, methane, carbon dioxide, carbon monoxide, ammonia or sulfur dioxide. The changes expected from clear, thick atmospheres containing enough of these gases were also absent. This does not mean that not a single molecule exists anywhere on the planet. It means that the observations did not show a mark large enough to detect.
No clear marks from gases appear in the current observations. Clear, thick atmospheres that should produce large marks are ruled out, but thin or cloud-covered atmospheres remain possible.
What environments could explain the lack of clear gas signatures?
First, an almost airless rocky surface would produce a flat spectrum because no gas absorbs extra light during transit. Second, an atmosphere thinner than Mars’s, or one made of heavy molecules, could be too low and compact for its absorption marks to be found at the present precision. Third, high clouds or haze could hide a broad range of wavelengths and flatten the molecular marks of gases below.
These three cases have very different surface temperatures and pressures, but can look alike in starlight measured during transit. Researchers need to combine more transits to sharpen small changes, measure how heat moves between day and night using the planet’s infrared light, and look for other marks that remain visible above clouds.
- Size and mass were determined Blocked starlight gave the size, while changes in neighboring planets’ transit times gave the mass.
- Changes made by the star were calculated Researchers examined how starspots and bright regions changed the color of the starlight.
- Several atmospheres were compared They predicted marks for different gases, pressures and cloud heights.
- Atmospheres that did not fit were excluded Hydrogen-rich or clear, thick atmospheres that should make obvious marks did not match the data.
- Three possibilities remained An almost airless surface, a very thin atmosphere and an atmosphere with high clouds need further study.
What does being near the habitable zone actually mean?
Planet d receives about as much energy as Earth does from the Sun, or slightly more. It lies near the star-facing edge of a broadly defined habitable zone. This means only that it is at a distance where liquid water could be possible with an atmosphere that holds the right amount of heat. It is not evidence that water, air, clouds or life have been found.
Without an atmosphere, the temperature difference between day and night would be large and surface water would be difficult to keep for long. With a thin atmosphere, the stability of liquid water would depend on its pressure and composition. With a thick atmosphere and high clouds, the greenhouse effect and reflectivity would change the temperature again. Current observations cannot choose among these environments.
The surface and clouds shown on this page are not an observed photograph. Webb measured whether the amount of light blocked changed with wavelength while the planet crossed in front of its star. Failing to find marks from gases does not mean that the planet has no atmosphere or life.
Sources
- NASA Exoplanet Archive — TRAPPIST-1 d
- NASA Science — Atmospheric Possibilities for TRAPPIST-1 d
- The Astrophysical Journal — NIRSpec Transmission Spectrum of TRAPPIST-1 d
- The Planetary Science Journal — TRAPPIST-1 Masses and Radii
- NASA Science — Webb Results for TRAPPIST-1
- NASA Science — TRAPPIST-1 d Catalog
- Nature — Seven Temperate Terrestrial Planets