A system of seven exoplanets — three in their star’s habitable zone — has been found by an international team of astronomers.

Using ground- and space-based telescopes, the planets were all detected as they passed in front of their parent star, TRAPPIST-1.

“We report the results of an intensive photometric monitoring campaign of that star from the ground and with NASA’s Spitzer Space Telescope,” the astronomers said.

“Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1.”

Also known as 2MASS J23062928-0502285, TRAPPIST-1 is an ultracool dwarf star in the constellation Aquarius, 38.8 light-years away.

It is barely larger than Solar System’s Jupiter and has just 8% of our Sun’s mass.

Astronomers expected that such dwarf stars might host many Earth-size planets in tight orbits, but TRAPPIST-1 is the first such planetary system to be found.

“The TRAPPIST-1 system is a compact analog of the inner Solar System,” the researchers said.

“It represents a unique opportunity to thoroughly characterize temperate Earth-like planets orbiting a much cooler and smaller star than the Sun, and notably to study the impact of tidal locking, tidal heating, stellar activity and an extended pre-main-sequence phase on their atmospheric properties.”

All seven planets in this system — TRAPPIST-1b, c, d, e, f, g and h — are similar in size to Earth and Venus, or slightly smaller. At least the innermost six are probably rocky in composition.

“This is an amazing planetary system — not only because we have found so many planets, but because they are all surprisingly similar in size to the Earth,” said team member Dr. Michaël Gillon, from the STAR Institute at the University of Liège in Belgium.

These planets have short orbital periods: 1.51, 2.42, 4.04, 6.06, 9.21, 12.35 and 20 days, respectively.

Their orbits are not much larger than that of Jupiter’s Galilean moon system, and much smaller than the orbit of Mercury in the Solar System.

However, TRAPPIST-1’s small size and low temperature mean that the energy input to its planets is similar to that received by the inner planets in our Solar System — TRAPPIST-1c, d and f receive similar amounts of energy to Venus, Earth and Mars, respectively.

“The energy output from dwarf stars like TRAPPIST-1 is much weaker than that of our Sun,” said team member Dr. Amaury Triaud, from Cambridge’s Institute of Astronomy.

“Planets would need to be in far closer orbits than we see in the Solar System if there is to be surface water.”

“Fortunately, it seems that this kind of compact configuration is just what we see around TRAPPIST-1.”

According to the team, all TRAPPIST-1 planets could potentially have liquid water on their surfaces, though their orbital distances make some of them more likely candidates than others.

Climate models suggest the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces.

The orbital distance of the system’s outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbor liquid water — assuming no alternative heating processes are occurring.

TRAPPIST-1e, f, and g, however, represent the holy grail for planet-hunting astronomers, as they orbit in the star’s habitable zone and could host oceans of surface water.

A paper reporting this discovery is published in the journal Nature.

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Michaël Gillon et al. Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1. Nature, published online February 22, 2017;