TRAPPIST-1

TRAPPIST-1 is a red dwarf star with seven known planets. It lies in the constellation Aquarius approximately 40.66 light-years away from Earth. An ultra-cool dwarf, it has a surface temperature of about 2,566 K (2,290 °C; 4,160 °F). Its radius is slightly larger than Jupiter's and it has a mass of about 9% of the Sun. It is estimated to be 7.6 billion years old, making it older than the Solar System. The discovery of the star was first published in 2000.

Observations in 2016 from TRAPPIST–South (Transiting Planets and Planetesimals Small Telescope project) at La Silla Observatory in Chile and other telescopes led to the discovery of two terrestrial planets in orbit around TRAPPIST-1. In 2017, further analysis of the original observations identified five more terrestrial planets. The seven planets take between 1.5 and 19 days to orbit the star in circular orbits. They are all likely tidally locked to TRAPPIST-1, and it is believed that each planet is in permanent day on one side and permanent night on the other. Their masses are comparable to that of Earth and they all lie in the same plane; seen from Earth, they pass in front of the star. This placement allowed the planets to be detected: when they pass in front of the star, its apparent magnitude dims.

Up to four of the planets—designated d, e, f, and g—orbit at distances where temperatures are likely suitable for the existence of liquid water, and are thus potentially hospitable to life. There is no evidence of an atmosphere on any of the planets, and observations of TRAPPIST-1b have in particular ruled out the existence of an atmosphere. It is unclear whether radiation emissions from TRAPPIST-1 would allow for such atmospheres. The planets have low densities; they may consist of large amounts of volatile material. Due to the possibility of several of the planets being habitable, the system has drawn interest from researchers and has appeared in popular culture.

Discovery

The star known as TRAPPIST-1 was discovered in 1999 by astronomer John Gizis and colleagues during a survey of close-by ultra-cool dwarf stars. It appeared in sample C of the surveyed stars, which was obtained in June 1999. Publication of the discovery took place in 2000. TRAPPIST-1 is named after TRAPPIST (the Transiting Planets and Planetesimals Small Telescope project), which discovered the first two exoplanets around the star.

Its planetary system was discovered by a team led by Michaël Gillon, a Belgian astronomer at the University of Liege, in 2016 during observations made at the La Silla Observatory, Chile, using the TRAPPIST telescope. The discovery was based on anomalies in the light curves measured by the telescope in 2015. These were initially interpreted as indicating the existence of three planets. In 2016, separate discoveries revealed that the third planet was in fact multiple planets. The telescopes and observatories involved were the Spitzer Space Telescope and the ground-based TRAPPIST–South, TRAPPIST–North in Oukaïmeden Observatory, Morocco, the South African Astronomical Observatory, and the Liverpool Telescopes and William Herschel Telescopes in Spain.

The observations of TRAPPIST-1 are considered among the most important research findings of the Spitzer Space Telescope. Complementing the findings were observations by the Himalayan Chandra Telescope, the United Kingdom Infrared Telescope, and the Very Large Telescope. Since then, research has confirmed the existence of at least seven planets in the system, the orbits of which have been calculated using measurements from the Spitzer and Kepler telescopes. Some news reports incorrectly attributed the discovery of the TRAPPIST-1 planets to NASA alone; in fact the TRAPPIST project that led to their discovery received funding from both NASA and the European Research Council of the European Union (EU).

Description

TRAPPIST-1 is in the constellation Aquarius, five degrees south of the celestial equator. It is a relatively close star located 40.66±0.04 light-years from Earth, with a large proper motion and no companion stars.

It is a red dwarf of spectral class M8.0±0.5, meaning it is relatively small and cold. With a radius 12% of that of the Sun, TRAPPIST-1 is only slightly larger than the planet Jupiter (though much more massive). Its mass is approximately 9% of that of the Sun, being just sufficient to allow nuclear fusion to take place. TRAPPIST-1's density is unusually low for a red dwarf. It has a low effective temperature of 2,566 K (2,293 °C) making it, as of 2022, the coldest-known star to host planets. TRAPPIST-1 is cold enough for condensates to form in its photosphere; these have been detected through the polarisation they induce in its radiation during transits of its planets. Elements heavier than helium form compounds in its atmosphere, which display as absorption lines in TRAPPIST-1's spectrum.

There is no evidence that it has a stellar cycle. Its luminosity, emitted mostly as infrared radiation, is about 0.055% that of the Sun. Low-precision measurements from the XMM-Newton satellite and other facilities show that the star emits faint radiation at short wavelengths such as X-rays and UV radiation. There are no detectable radio wave emissions.

Rotation period and age

Measurements of TRAPPIST-1's rotation have yielded a period of 3.3 days; earlier measurements of 1.4 days appear to have been caused by changes in the distribution of its starspots. Its rotational axis may be slightly offset from that of its planets.

Using a combination of techniques including composition and movements of the star, the age of TRAPPIST-1 has been estimated at about 7.6±2.2 billion years, making it older than the Solar System, which is about 4.5 billion years old. It is expected to shine for ten trillion years—about 700 times longer than the present age of the Universe—whereas the Sun will run out of hydrogen and leave the main sequence in a few billion years.

Activity

Photospheric features have been detected on TRAPPIST-1 and probably make up a substantial part of the stellar surface. James Webb Space Telescope (JWST) observations indicate that cold starspots might cover up to one quarter of its photosphere. The Kepler and Spitzer Space Telescopes have observed possible bright spots, which may be faculae, although some of these may be too large to qualify as such. Bright spots are correlated to the occurrence of some stellar flares. and their energy distribution resembles that of solar flares. Kepler K2 observations have shown that TRAPPIST-1 produces frequent flares (42 flares in 80 days), including large, complex flares that could alter nearby planetary atmospheres irreversibly and significantly, raising doubts of hosting life as we know it on Earth.

The star has a strong magnetic field with a mean intensity of about 600 gauss which may be an underestimate. The magnetic field drives high chromospheric activity, and may be capable of trapping coronal mass ejections.

According to Garraffo et al. (2017), TRAPPIST-1 loses about 3×10⁻¹⁴ solar masses per year to the stellar wind, a rate which is about 1.5 times that of the Sun. Dong et al. (2018) simulated the observed properties of TRAPPIST-1 with a mass loss of 4.1×10⁻¹⁵ solar masses per year. Simulations to estimate mass loss are complicated because, as of 2019, most of the parameters that govern TRAPPIST-1's stellar wind are not known from direct observation.

Planetary system

TRAPPIST-1 is orbited by seven planets, designated TRAPPIST-1b, 1c, 1d, 1e, 1f, 1g, and 1h in alphabetic order going out from the star. These planets have orbital periods ranging from 1.5 to 19 days, at distances of 0.011–0.059 astronomical units (1.7–8.9 million km).

All the planets are much closer to their star than Mercury is to the Sun, with the distance between TRAPPIST-1b and 1c at syzygy being only twice that between the Earth and Moon, making the TRAPPIST-1 system very compact. Kral et al. (2018) did not detect any comets around TRAPPIST-1, and Marino et al. (2020) found no evidence of a Kuiper belt, although it is uncertain whether a Solar System-like belt around TRAPPIST-1 would be observable from Earth. Observations with the Atacama Large Millimeter Array found no evidence of a circumstellar dust disk.

The inclinations of planetary orbits relative to each other are less than 0.1 degrees, making TRAPPIST-1 the flattest planetary system in the NASA Exoplanet Archive. The orbits are highly circular, with minimal eccentricities and are well-aligned with the spin axis of TRAPPIST-1. The planets orbit in the same plane and, from the perspective of the Solar System, transit TRAPPIST-1 during their orbit and frequently pass in front of each other.