Three planets orbiting an ultracool dwarf star just 40 light-years from Earth have been discovered by Astronomers using the TRAPPIST telescope at the European Southern Observatory’s La Silla Observatory.
The worlds have sizes and temperatures similar to those of Venus and Earth and are the best targets found so far for the search for life outside the Solar System. They are also the first planets ever discovered around such a tiny and dim star.
The Belgian TRAPPIST telescope was used to observe the star 2MASS J23062928-0502285, now also known as TRAPPIST-1. They found that this dim and cool star faded slightly at regular intervals, indicating that several objects were passing between the star and the Earth.
Detailed analysis showed that three exoplanets with similar sizes to the Earth were present.
“This really is a paradigm shift with regards to the planet population and the path towards finding life in the Universe. So far, the existence of such ‘red worlds’ orbiting ultra-cool dwarf stars was purely theoretical, but now we have not just one lonely planet around such a faint red star but a complete system of three planets,”
Emmanuël Jehin, a co-author of the new study, said.
Ultracool Dwarf Star
TRAPPIST-1, an ultracool dwarf star, is much cooler and redder than the Sun and barely larger than Jupiter. Such stars are both very common in the Milky Way and very long-lived, but this is the first time that planets have been found around one of them.
Despite being so close to the Earth, this star is too dim and too red to be seen with the naked eye or even visually with a large amateur telescope. It lies in the constellation of Aquarius (The Water Carrier).
Michaël Gillon, lead author of the paper presenting the discovery, explains the significance:
“Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighbourhood? The reason is simple: systems around these tiny stars are the only places where we can detect life on an Earth-sized exoplanet with our current technology. So if we want to find life elsewhere in the Universe, this is where we should start to look.”
Follow-up observations with larger telescopes, including the HAWK-I instrument on ESO’s 8-metre Very Large Telescope in Chile, have shown that the planets orbiting TRAPPIST-1 have sizes very similar to that of Earth. Two of the planets have orbital periods of about 1.5 days and 2.4 days respectively, and the third planet has a less well determined period in the range 4.5 to 73 days.
“With such short orbital periods, the planets are between 20 and 100 times closer to their star than the Earth to the Sun. The structure of this planetary system is much more similar in scale to the system of Jupiter’s moons than to that of the Solar System,”
explains Michaël Gillon.
TRAPPIST (the TRAnsiting Planets and PlanetesImals Small Telescope) is a Belgian robotic 0.6-metre telescope operated from the University of Liège and based at ESO’s La Silla Observatory in Chile.
It spends much of its time monitoring the light from around 60 of the nearest ultracool dwarf stars and brown dwarfs (“stars” which are not quite massive enough to initiate sustained nuclear fusion in their cores), looking for evidence of planetary transits. The target, in this case, TRAPPIST-1, is an ultracool dwarf, with about 0.05% of the Sun’s luminosity and a mass of about 8% that of the Sun.
This work opens up a new direction for exoplanet hunting, as around 15% of the stars near the Sun are ultra-cool dwarf stars, and it also serves to highlight that the search for exoplanets has now entered the realm of potentially habitable cousins of the Earth.
Although they orbit very close to their host dwarf star, the inner two planets only receive four times and twice, respectively, the amount of radiation received by the Earth, because their star is much fainter than the Sun. That puts them closer to the star than the habitable zone for this system, although it is still possible that they possess habitable regions on their surfaces.
The third, outer, planet’s orbit is not yet well known, but it probably receives less radiation than the Earth does, but maybe still enough to lie within the habitable zone.
The TRAPPIST survey is a prototype for a more ambitious project called the Search for habitable Planets EClipsing ULtra-cOOl Stars (SPECULOOS) that will be installed at ESO’s Paranal Observatory. Four 1-metre robotic telescopes will be installed at the Paranal Observatory to search for habitable planets around 500 ultra-cool stars over the next five years.
Study: Temperate Earth-sized planets transiting a nearby ultracool dwarf star