registers the light reduction. The first
observations showed that two Earth-like
planets were orbiting close to the star,
whereas yet another planet was orbiting
further away. The star was named
Trappist-1, and the Belgian astronomers
were allowed to use the Spitzer space
telescope for 500 hours of further studies.
The observations paid off. The telescope
revealed that a total of seven rocky planets
orbit the dwarf star.
Five of the planets are the size of Earth,
whereas the two last ones are smaller, sized
somewhere in between Earth and Mars.
The seven are orbiting the star so close
together that the distance between the two
innermost planets is only 1.6 times the
distance between Earth and the Moon. The
innermost planet only takes 1.5 days to
complete an orbit, whereas the outermost
planet orbits the star in a matter of 19 days.
In comparison, the innermost planet of
the Solar System, Mercury, takes 88 days to
orbit the Sun.STAR SWALLOWED THE FIRST PLANETS
The tiny system is only slightly bigger than
Jupiter and its four largest moons – Io,
Europa, Ganymede, and Callisto. The
proximity of the dwarf star means that three
of the planets might include life. The red
dwarf only shines 0.05 % as brightly as the
Sun, but that is more than enough to make
the three innermost planets too hot. Thethree next ones are located in the habitable
zone, where liquid water could exist. The
outermost planet is probably frozen.
The great similarity to Jupiter’s moons
has convinced the Belgian astronomers that
the planetary system was formed in the
same way as they were. Consequently, the
newborn dwarf star was surrounded by a disc
of dust and gas, which was so dense that it
gave birth to several generations of planets.
Together with lots of dust, they spiralled into
the star and were swallowed one by one. Not
until the disc had become so "thin" that the
gravity of the dust no longer contributed to
attracting the planets to the star, the
existing planets were formed. At the same
time, the planets’ mutual attractionPLANET E COULD HAVE
OCEANS AND LIFE
The temperatures on the planet
allow for liquid water on the
surface, where different life forms
could thrive.
DISTANCE TO THE STAR: 0.028 AU
ORBIT TIME: 6.1 days
MASS: 0.62 Earth masses
RADIUS: 0.92 times Earth's
IRRADIATION: 0.66 times the
solar irradiation on Earth
PLANET F IS COLDER
THAN EARTH
The planet receives less than half
the heat that Earth gets from the
Sun, but a water cycle - vital for
life - could easily exist.DISTANCE TO THE STAR: 0.037 AU
ORBIT TIME: 9.21 days
MASS: 0.68 Earth masses
RADIUS: 1.04 times Earth's
IRRADIATION: 0.38 times the
solar irradiation on EarthPLANET G IS THE
SY STEM'S LARGEST
On the outskirts of the habitable
zone, you will find the largest
planet with a radius of 1.13 times
Earth's. Could life survive there?DISTANCE TO THE STAR: 0.045 AU
ORBIT TIME: 12.35 days
MASS: 1.34 Earth masses
RADIUS: 1.13 times Earth's
IRRADIATION: 0.26 times the
solar irradiation on EarthPLANET H IS TOO COLD
The star shines so weakly that
the planet is probably frozen, so
the chances of life are minimal.DISTANCE TO THE STAR:
0.06 AU
ORBIT TIME: 19 days
MASS: Unknown
RADIUS: 0.76 times Earth'sTHE HABITABLE ZONE
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