Since the discovery of the first exoplanet in the 1990s, we have been looking for planets similar to Earth. Well, today is our lucky day: We have now found three of them around the same star and they are close enough to study in detail. These planets could be our best bet so far for finding life outside the Solar System.

Belgian astronomers discovered the three potentially habitable Earth-like planets orbiting an ultracool dwarf star, named TRAPPIST-1, about 40 light-years from Earth. These are some of the smallest exoplanets ever discovered, with a radius only slightly bigger than Earth's, and they are the first planets discovered around an ultracool dwarf – a dim star not much bigger than Jupiter. Their results are published in Nature.

The most exciting thing about this discovery, made by the Belgian TRAPPIST telescope as the planets passed in front of the star, is that these planets are close enough for us to study. Many of the other potentially habitable worlds we've found are much further away and around much brighter stars, making them more difficult to observe.

“These are the first planets similar in size and temperature to Earth and Venus for which we can study the atmospheric composition in detail, and really constrain the surface conditions and habitability,” lead author Dr. Michaël Gillon from the University of Liège told IFLScience.

This artist’s impression shows an imagined view from the surface of TRAPPIST-1d, the furthest of the three exoplanets. ​ESO/M. Kornmesser

At first glance, the system might not seem that promising. Two of the planets, TRAPPIST- 1b and c, have years lasting about 1.5 and 2.4 Earth days respectively, meaning they orbit very closely, while TRAPPIST-1d has a less well determined period in the range of 4.5 to 73 days. However, the star has a surface temperature of only 2,550 Kelvins (2,277°C / 4,130 °F ), so they are unlikely to be inferno worlds. Instead, they could be quite the opposite.

The closest two planets receive no more than four times the amount of radiation received by Earth, while the furthest planet likely receives less. This puts the planets at the edges of the star's habitable zone, the region in which liquid water can exist. It's not certain if the planets are solid, but TRAPPIST-1 is rich in heavy elements, which indicates a suitable evironment for rocky planets to form.

The planets have a radius of 1.11, 1.05, and 1.16 times that of Earth, which combined with their locations strongly indicates that these objects possess some of the right conditions for life. The planets’ temperatures could range from slightly higher than water’s boiling point to well below freezing.

An artist's impression of TRAPPIST-1 compared to our Sun. ESO

Although the investigation is groundbreaking, there are still many unknowns about the nature of these planets, such as their mass or what they are made of. But it's a very promising discovery, especially as these are the first planets found around this type of star.

Ultracool dwarfs are a diverse class of stars that encompass both the lowest mass stars and brown dwarfs, objects that don't quite make it into stars. TRAPPIST-1 is one of the lowest mass stars, and its luminosity – which is a measly 0.05 percent of the Sun's – will not change for tens of billions of years, providing a very stable environment for the planets. Of course, the star's dim light also means we can see the planets more easily.

“Why are we trying to detect Earth-like planets around the smallest and coolest stars in the solar neighborhood?” Dr. Gillon said in a statement. “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.”

Hubble should be able to provide some preliminary constraints on the planets’ atmospheric compositions, while its successor, the James Webb Space Telescope (JWST), could confirm the temperatures and maybe even the presence of biological molecules. The team will update a website so anyone can follow the advancement of the system's exploration.

While there are still many unknowns about this system, astronomers are confident that we will soon be able to know more about these planets. One thing is certain though – Earth is not that special anymore.