Our Spitzer Space
Telescope has revealed the first known system of seven Earth-size planets
around a single star. Three of these planets are firmly located in an area
called the habitable zone, where liquid water is most likely to exist on a
rocky planet.
Assisted
by several ground-based telescopes, Spitzer confirmed the existence of two of
these planets and discovered five additional ones, increasing the number of
known planets in the system to seven.
This is
the FIRST time three terrestrial
planets have been found in the habitable zone of a star, and this is the FIRST time we have been able to measure
both the masses and the radius for habitable zone Earth-sized planets.
All of
these seven planets could have liquid water, key to life as we know it, under
the right atmospheric conditions, but the chances are highest with the three in
the habitable zone.
At about
40 light-years (235 trillion miles) from Earth, the system of planets is
relatively close to us, in the constellation Aquarius. Because they are located
outside of our solar system, these planets are scientifically known as exoplanets. To clarify, exoplanets are
planets outside our solar system that orbit a sun-like star.
In this
animation, you can see the planets orbiting the star, with the green area
representing the famous habitable zone, defined as the range of distance to the
star for which an Earth-like planet is the most likely to harbor abundant
liquid water on its surface. Planets e, f and g fall in the habitable zone of
the star.
Using
Spitzer data, the team precisely measured the sizes of the seven planets and
developed first estimates of the masses of six of them. The mass of the seventh
and farthest exoplanet has not yet been estimated.
For
comparison…if our sun was the size of a basketball, the TRAPPIST-1 star would
be the size of a golf ball.
Based on
their densities, all of the TRAPPIST-1 planets are likely to be rocky. Further
observations will not only help determine whether they are rich in water, but
also possibly reveal whether any could have liquid water on their surfaces.
The sun at
the center of this system is classified as an ultra-cool dwarf and is so cool
that liquid water could survive on planets orbiting very close to it, closer
than is possible on planets in our solar system. All seven of the TRAPPIST-1
planetary orbits are closer to their host star than Mercury is to our sun.
The
planets also are very close to each other. How close? Well, if a person was
standing on one of the planet’s surface, they could gaze up and potentially see
geological features or clouds of neighboring worlds, which would sometimes
appear larger than the moon in Earth’s sky.
The
planets may also be tidally-locked to their star, which means the same side of
the planet is always facing the star, therefore each side is either perpetual
day or night. This could mean they have weather patterns totally unlike those
on Earth, such as strong wind blowing from the day side to the night side, and
extreme temperature changes.
Because most
TRAPPIST-1 planets are likely to be rocky, and they are very close to one
another, scientists view the Galilean moons of Jupiter – lo, Europa, Callisto,
Ganymede – as good comparisons in our solar system. All of these moons are also
tidally locked to Jupiter. The TRAPPIST-1 star is only slightly wider than
Jupiter, yet much warmer.
How Did the Spitzer Space Telescope Detect this System?
Spitzer,
an infrared telescope that trails Earth as it orbits the sun, was well-suited
for studying TRAPPIST-1 because the star glows brightest in infrared light,
whose wavelengths are longer than the eye can see. Spitzer is uniquely
positioned in its orbit to observe enough crossing (aka transits) of the
planets in front of the host star to reveal the complex architecture of the
system.
Every time a planet passes by, or transits, a star, it blocks out some
light. Spitzer measured the dips in light and based on how big the dip, you can
determine the size of the planet. The timing of the transits tells you how long
it takes for the planet to orbit the star.
The
TRAPPIST-1 system provides one of the best opportunities in the next decade to
study the atmospheres around Earth-size planets. Spitzer, Hubble and Kepler will
help astronomers plan for follow-up studies using our upcoming James Webb Space
Telescope, launching in 2018. With much greater sensitivity, Webb will be
able to detect the chemical fingerprints of water, methane, oxygen, ozone and
other components of a planet’s atmosphere.
At 40 light-years away, humans won’t be visiting this system in person anytime soon…that said…this poster can help us imagine what it would be like:
