Astronomers have discovered three planets orbiting the habitable zone of an ultra-cool dwarf star just 40 light-years from Earth. The discovery is the first evidence supporting the hypothesis that these very low-mass and low-temperature stars should have Earth-sized or smaller planets orbiting them. The planets, which are the size of Venus and Earth, are currently the best places to look for life outside our solar system, said the team of researchers led by astronomer Michaël Gillon, from the University of Liege in Belgium.

The planets were found almost by accident, as an international group of astronomers were conducting a test survey of an ultra-cool dwarf star labelled TRAPPIST-1, which lies in the constellation of Aquarius.

The star, TRAPPIST-1, is about the size of Jupiter but is one thousand times dimmer than our Sun and shines in the much cooler infrared part of the light spectrum. Orbiting it are three planets; two orbiting within 1.1 to 1.5 per cent of the distance between the Earth and the Sun and passing in front of the star every one to two days.

NOTE

• The researchers used a small 60-centimeter telescope to monitor the brightness of TRAPPIST-1 every 1.2 minutes for 245 hours over 62 nights.
• The small telescope picked up the periodic dimming of the star, which suggested planets were passing in front of it and blocking some of its light.
• The research team then used three much larger telescopes in India, Chile and Hawaii to confirm that these were indeed planets orbiting the star.