What makes TRAPPIST-1 helpful—and challenging—for planet observations?
TRAPPIST-1 has about 12% of the Sun’s radius and is only slightly larger than Jupiter. Because the star is small, an Earth-sized planet passing in front of it blocks about 0.6% of its light. That change is roughly 70 times greater than the one produced if the same planet crossed the Sun, making the planet’s size and atmosphere easier to observe. Yet the star’s surface contains dark spots and bright regions, and it also produces flares. When the star’s own light changes over time and at different wavelengths, those changes can mix with the marks left by a planet’s atmosphere.
TRAPPIST-1’s small size helped astronomers find its seven planets, but studying each atmosphere requires a detailed understanding of the changing star as well. If researchers assume that every part of its surface shines alike, they may appear to detect a gas that is not on the planet or mistake a real atmospheric signal for a change in the star.
- About 40 light-years Distance from the solar system to TRAPPIST-1
- About 12% of the Sun’s radius A star only slightly larger than Jupiter
- About 9% of the Sun’s mass Enough mass to keep hydrogen fusion running in its core
- About 2,566 K A surface temperature of roughly 2,300°C
Why is TRAPPIST-1 a star even though it is similar in size to Jupiter?
Astronomers classify TRAPPIST-1 as an M8V ultracool red dwarf. “Ultracool” means cool compared with other stars. Its surface temperature is about 2,566 K, or roughly 2,300°C—less than half the Sun’s temperature but still hot enough to melt rock and metal. Most of the star’s light emerges as infrared radiation, at wavelengths longer than visible light.
Although it is similar in size to Jupiter, it has about 94 times Jupiter’s mass. Much more material is packed into a similar volume, making the temperature and pressure at its center high enough to sustain fusion that turns hydrogen into energy. That distinguishes it from planets and brown dwarfs that cannot maintain hydrogen fusion. TRAPPIST-1 is a small star close to the minimum mass needed to be a star.
Why are planets easier to find around a small star?
The fraction of starlight blocked by a planet depends on the difference between the sizes of the planet and star. TRAPPIST-1 has about 13 times Earth’s radius, so one Earth-sized planet covers roughly one part in 170 of the star’s round disk. Its light falls by about 0.6%. That is about 70 times the change produced when Earth crosses the Sun.
The star is small, and its seven planets complete their orbits quickly—every 1.5 to 18.8 days—so they cross it often. This helped the ground-based TRAPPIST telescope and Spitzer confirm all seven, and it lets Webb combine repeated observations to investigate their atmospheres. A large blocked fraction does not mean the star itself is bright. TRAPPIST-1 is much dimmer than the Sun, so many observations must be combined to distinguish small changes reliably.
How can we find a planet’s atmosphere in starlight?
When a planet crosses its star, some starlight passes through the atmosphere around the planet’s edge. Gases absorb particular wavelengths, making the planet appear very slightly larger in those parts of the spectrum. Researchers split the starlight by wavelength and compare it when the planet is away from the star’s face with the light recorded during the transit. The result is called a transmission spectrum.
This method assumes that the starlight at the two times was originally the same. The comparison would be simple if the entire surface had one temperature, but TRAPPIST-1’s surface is uneven. If the region crossed by the planet differs in temperature and brightness from the uncovered surface, the measured difference includes both the planet’s atmosphere and the star’s surface variations.
- Measure the starlight when the planet is not crossing Separate the light by wavelength and record its brightness.
- Measure it again during the transit Record in the same way the light left after the planet and its atmosphere have blocked part of it.
- Compare the two measurements Calculate which wavelengths the planet blocked more strongly.
- Remove changes caused by the star Model how starspots, bright regions and flares altered the starlight.
- Check whether the signal repeats Consider a gas in the atmosphere only when the same mark appears in multiple observations.
Can changes on a star’s surface look like a planetary atmosphere?
Starspots are cooler and darker than their surroundings, while bright regions are hotter. Even if a planet never crosses them directly, the star’s ordinary light changes with the fraction of its surface covered by each kind of region. Cool spots darken different wavelengths by different amounts and can create a shape that resembles absorption by water or another gas in a planet’s atmosphere.
If a planet crosses a starspot, it blocks less light and the observed brightness briefly rises; if it crosses a bright region, the star can dim more than usual. Flares also change ultraviolet, visible and infrared brightness together over short periods. Because their timing and effect at each wavelength differ, one observation cannot easily separate all of them. Webb’s first observations of TRAPPIST-1 b likewise showed features that appeared to be caused by spots or bright regions and changed from one observing visit to another.
A planet blocks a larger share of light in front of a small star. But measuring its atmosphere accurately also requires accounting for the brightness difference between the parts the planet covers and leaves uncovered.
How can we tell the star’s signal from the planet’s?
A single transit cannot reliably separate changes created by the star from the marks of a planet’s atmosphere. Researchers compare transits from different times and record the star’s brightness and flares before and after each one. Observing two planets crossing in quick succession can also provide two data sets before the stellar surface changes greatly, helping identify the star’s contribution.
Measuring how much the total light falls when a planet passes behind the star reveals heat from the planet’s dayside. That information differs from what is measured when the planet passes in front. Researchers must compare whether different wavelengths and observing methods point to the same atmospheric explanation. NASA says that, depending on the planet, distinguishing an atmosphere with confidence could require hundreds of transits over several years. This is an estimate of the difficulty caused by the changing star, not a fixed number of scheduled observations.
Can an old star still produce flares?
TRAPPIST-1 is estimated to be about 7.6 billion years old, with an uncertainty of roughly 2.2 billion years in either direction. It is older than the Sun, but old age does not mean that magnetic activity has disappeared. Red dwarfs change slowly and can retain magnetic activity and flares for a very long time.
The star is faint, so its planets must orbit close by to receive a moderate amount of starlight. All seven are closer to their star than Mercury is to the Sun. Ultraviolet light and X-rays from flares can change atmospheric molecules on nearby planets and help some gases escape into space. But the presence of flares alone does not prove that a particular planet has lost its atmosphere. The outcome depends on how thick the atmosphere was initially and whether a magnetic field or gases released from the interior can replenish it.
The star image on this page is neither a detailed photograph of TRAPPIST-1’s surface nor its true color as a person would see it. The dark and bright areas were illustrated to explain changes in starlight. Observations establish the star’s overall size, mass and temperature, and how its light changes with time.
Sources
- NASA Exoplanet Archive — TRAPPIST-1
- NASA Science — What Is Webb Revealing About the TRAPPIST-1 System?
- NASA Science — Webb Measures the Temperature of TRAPPIST-1 b
- NASA Science — TRAPPIST-1 Is Older Than Our Solar System
- Nature — Seven Temperate Terrestrial Planets Around TRAPPIST-1
- The Astrophysical Journal Letters — Strong Stellar Contamination in TRAPPIST-1 b Spectra
- The Astronomical Journal — Back-to-back Transit Stellar Correction