TRAPPIST-1 is a red dwarf star about 40 light-years from Earth in the constellation Aquarius. One light-year equals the distance light travels in a vacuum in a year, about 5.88 trillion miles (9.46 trillion kilometers). TRAPPIST-1 is notable for having seven orbiting planets. Astronomers classify the planets as terrestrial, meaning they have Earthlike qualities. Three of the planets orbit the star within a region that astronomers call the habitable zone. In that region, liquid water can exist on a planet’s surface. Scientists consider liquid water to be an essential ingredient for life.
TRAPPIST-1 is a small, cool star. It has a diameter only about 1/10 that of our sun—about the size of the planet Jupiter. However, the star has a mass about 84 times Jupiter’s mass. Its temperature is about 2500 K compared with 5800 K for our sun. One kelvin, denoted by K, is measured in degrees Celsius above an absolute zero that occurs at –273.15 °C. Because of its lower temperature, TRAPPIST-1 has a luminosity (rate of energy output) only about 1/200 that of our sun. Scientists have determined that the star formed approximately 7.6 billion years ago. By comparison, our sun formed only 4.6 billion years ago.
Astrobiologists, scientists who search for and study life in the universe, can study the TRAPPIST-1 planetary system to investigate how a planet’s composition and its distance from a star affect the evolution of its habitability. In addition, the TRAPPIST-1 system might be much older than our solar system. Studying it may provide insights into how planetary systems evolve over such long periods of time.
Planetary system.
The TRAPPIST-1 planetary system contains seven closely orbiting terrestrial planets. Five are about the size of Earth. These are designated b, c, e, f, and g, in order of their discovery. Two of the planets, designated d and h, are midway in size between that of Mars and that of Earth. The TRAPPIST-1 planets orbit the star rapidly. The orbital period—the time it takes a planet to complete one orbit around the star—ranges from 1.5 to 18.8 Earth days. Earth’s orbital period, also known as a year, is 365 days.
The density of the individual planets in the TRAPPIST-1 system ranges from about 60 percent to 95 percent of Earth’s density. From this finding, scientists have determined the planets are mostly rocky and metallic in composition, like Earth. The density of some of the planets suggests they could have a substantial ocean of water.
Three of the TRAPPIST-1 planets, e, f, and g, orbit within the habitable zone of the star. Thus, Earthlike life could exist or have existed on these three planets. By comparison, in our solar system only Earth is in the habitable zone. Venus orbits just inside the inner boundary of the zone and Mars orbits outside the outer boundary.
Other characteristics of a planetary system may affect the potential for life. TRAPPIST-1 is an active star that occasionally produces much higher fluxes of X rays and ultraviolet light than are emitted by our sun. The radiation emitted from the star might be harmful to life and could have caused a large amount of water to escape from some of the planets over their history. However, even a shallow ocean would shield any living things from the harmful effects of TRAPPIST-1’s radiation. A layer of ozone in the atmosphere, like that surrounding Earth, would also provide protection for any living things.
History of study.
The first three planets in the TRAPPIST-1 system were discovered in 2015. They were discovered by astronomers using the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) robotic telescope pair, located at La Silla Observatory in Chile and Oukaïmeden Observatory in Morocco. Scientists using the Spitzer Space Telescope and the Very Large Telescope in Chile announced in 2017 that they had confirmed the existence of those three planets and had discovered four more planets.
Because the TRAPPIST-1 planetary system is relatively close to Earth, scientists believe it is a good place to search for signs of life outside our solar system. The James Webb Space Telescope (JWST) studies the atmospheres of the TRAPPIST-1 planets and other exoplanets. JWST could detect in these atmospheres chemicals that are associated with life, if such chemicals are present. On Earth, life has altered the composition of the atmosphere through the release of certain gases, such as oxygen and methane. JWST has found that TRAPPIST-1 planets b and c probably have no atmosphere, or only an extremely thin one.
