When we think about planets outside our solar system, the TRAPPIST-1 star system often comes to mind. This intriguing system features a red dwarf star, TRAPPIST-1, accompanied by seven orbiting planets. The name TRAPPIST comes from the telescope that discovered the star.

The Transiting Planets and Planetesimals Small Telescope (TRAPPIST) refers to two robotic telescopes from Belgium: TRAPPIST–South, operating in the Chilean mountains at ESO’s La Silla Observatory since 2010, and TRAPPIST–North, operational since 2016 at the Oukaïmeden Observatory in the Atlas Mountains of Morocco. These telescopes are used to find and study planets outside our solar system and to observe comets around the Sun. The name honors Belgium’s famous Trappist beers.

 The Discovery of TRAPPIST-1

Astronomer John Gizis and his team discovered TRAPPIST-1 in 1999 while surveying nearby ultra-cool dwarf stars. The discovery, published in 2000, identified TRAPPIST-1 among a group of stars studied in June 1999. The star was named after the TRAPPIST project, which later discovered its first two exoplanets.

In 2016, Belgian astronomer Michaël Gillon and his team at the University of Liège discovered the star’s planetary system using the TRAPPIST telescope at La Silla Observatory. They observed changes in the star’s light in 2015, suggesting the presence of three planets. Further analysis revealed that the third “planet” was actually multiple planets. The discovery involved several telescopes, including the Spitzer Space Telescope, TRAPPIST, TRAPPIST-North, the South African Astronomical Observatory, and the Liverpool and William Herschel Telescopes in Spain.

 Characteristics of the TRAPPIST-1 System

TRAPPIST-1 lies in the constellation Aquarius, approximately 40.66 light-years away from Earth. Despite its proximity, it is not visible to the naked eye due to its nature as a red dwarf star of spectral class M8.0. TRAPPIST-1 is small and cool, with a radius just 12% that of the Sun and a mass about 9% of the Sun’s. With a low density and a temperature of 2,566 K (2,293 °C), TRAPPIST-1 holds the record as the coldest star known to host planets as of 2022.

The TRAPPIST-1 star system is older than our solar system, with an estimated age of 7.6 billion years. In contrast, our solar system is about 4.5 billion years old. TRAPPIST-1 is anticipated to continue shining for approximately ten trillion years, which vastly exceeds the current age of the Universe.

 The Planets of TRAPPIST-1

The TRAPPIST-1 system’s seven planets are named TRAPPIST-1b, 1c, 1d, 1e, 1f, 1g, and 1h. These planets take between 1.5 and 19 days to orbit the star, at distances ranging from 1.7 million to 8.9 million kilometers (0.011 to 0.059 astronomical units). All the planets are much closer to TRAPPIST-1 than Mercury is to the Sun, making this system very compact. Their orbits are very flat and nearly circular, well-aligned with the star’s spin axis. The planets transit in front of TRAPPIST-1 during their orbits and frequently cross paths(Transist) with each other.

The planets’ sizes range between 77.5% and 112.9% of Earth’s radius, with mass ratios similar to those between the moons and gas giants in our solar system. The planets likely have similar compositions to Earth but are less dense, suggesting they contain lighter materials like water. Planets b, d, f, g, and h are expected to have a lot of volatile chemicals.

 Habitability Potential

The habitable zone of TRAPPIST-1 is much closer to the star than that of the Sun. Three or four of its planets—e, f, and g or d, e, and f—might be in this zone. This is the largest known number of planets in the habitable zone of any star system. Whether these planets have liquid water depends on factors like reflectivity, atmospheric presence, and greenhouse effects.

TRAPPIST-1b

TRAPPIST-1b, orbiting very close to the star, completes an orbit in 1.51 Earth days. It receives more than four times the radiation Earth gets, and its high temperature suggests it might have experienced a runaway greenhouse effect. Observations indicate it has little to no atmosphere.

TRAPPIST-1c

TRAPPIST-1c orbits its star every 2.42 Earth days. It is outside the habitable zone and receives twice as much radiation as Earth. Observations ruled out CO2-rich atmospheres but didn’t confirm if it has a water vapor or oxygen-rich atmosphere, or no atmosphere at all.

TRAPPIST-1d

TRAPPIST-1d, which orbits the star every 4.05 Earth days, is near the inner edge of the habitable zone. The presence of liquid water or a runaway greenhouse effect on this planet is contingent on its atmospheric conditions.

TRAPPIST-1e

TRAPPIST-1e, with a density similar to Earth’s, orbits every 6.10 Earth days. It is the most likely to have retained its water and might have liquid water in various climate states. It is in the habitable zone and could have water amounts similar to Earth’s oceans.

TRAPPIST-1f

TRAPPIST-1f, orbiting every 9.21 Earth days, is likely a snowball planet. It might have a subsurface ocean, with moderate CO2 levels or tidal heating creating liquid water in certain areas.

 TRAPPIST-1g

TRAPPIST-1g orbits every 12.4 Earth days and is also likely a snowball planet. It might have a subsurface ocean and could contain up to half water by mass.

TRAPPIST-1h

TRAPPIST-1h, the farthest planet, orbits every 18.9 Earth days. It is likely a snowball planet, possibly with a methane/nitrogen atmosphere like Titan. It might have a subsurface ocean, with large amounts of CO2, hydrogen, methane, or internal heat needed for liquid water to exist.

Conclusion

The TRAPPIST-1 star system offers an exciting glimpse into the potential for habitability outside our solar system. With its seven planets, several of which lie in the habitable zone, TRAPPIST-1 continues to intrigue scientists and capture the public’s imagination. As we learn more about these distant worlds, we come closer to understanding the possibilities for life beyond Earth.