Sky and Telescope - June 2017 - 13

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p STARLIGHT DIPS This plot shows how the brightness of the faint
dwarf star TRAPPIST-1 varied as three of its planets passed across
its face in a triple transit on December 11, 2015. Data come from the
HAWK-I instrument on ESO's Very Large Telescope. The cartoon below
the light curve shows possible configurations for the planets crossing in
front of the stellar disk at three times during the triple transit.

follow-up campaign, the observers have discovered that there
are actually seven planets, not three. All are likely rocky. At
least three lie in TRAPPIST-1's putative habitable zone - the
region where, given an Earth-like composition, liquid water
could be stable on the surface.

LE AH TISCIONE / S&T, ESO / M. GILLON E T A L. (2)

From Three to Seven
The discovery roller coaster began when the team found that
what it had thought was a combined transit of two planets
was in fact the crossing of three.
The astronomers next assailed TRAPPIST-1 with an
impressive ﬂurry of ground-based observations. But the big
breakthrough came with NASA's Spitzer Space Telescope,
which observed the star for 20 days. These data include 34
clear transits. Gillon and his colleagues were then able to
combine their ground- and space-based observations and
slice and dice them to determine that the various signals
likely came from seven different planets. (Incidentally, the
original third planet doesn't exist.)
Only six of those are ﬁrm detections, however. The seventh planet, designated h, is iffy in its specs: The team only
detected a single transit for it, and astronomers prefer to see
three transits before calling something a candidate planet.
Expect astronomers to haggle over this one.
But let's assume for now that all seven exoplanets are real.
All their orbits would easily ﬁt inside Mercury's circuit around
the Sun. The years of the inner six range from 1.5 to 12 Earth
days, with the period of outermost h being between 14 and 35
days - giving it an orbit less than 20% as large as Mercury's.

Based on their transits, the smallest two worlds are about
three-fourths the diameter of Earth, the largest 10% greater
than it. Transits don't reveal masses, but changes in their
timing can. One of the wonderful things about this system
is that the exoplanets have resonant orbits: Their orbital
periods are roughly integer ratios of one another, a setup that
gravitationally links the planets together and can lead to tiny
shifts in their positions - and the times of their transits.
Based on these shifts, the researchers calculated the planets'
shared gravitational inﬂuences, and hence their approximate
masses and densities. All are consistent with being rocky, the
team concludes in the February 23rd Nature.
Such resonant orbits arise when worlds migrate from their
original locations, Gillon explains. Astronomers think that
when lightweight planets form far out in a star's planetforming disk, gas drag and such will make them advance
inward. During this inbound migration, the worlds catch one
another in resonant orbits, such that they can form a "chain
of planets," he says. But it's unclear when that happened, or
whether these orbits are stable: The researchers haven't determined the seventh planet's path, nor do they know if there
are other worlds in the system mucking things up.
The planets are all likely tidally locked with their star,
meaning they always point the same hemisphere at it, as the
Moon does to Earth. So close to the star, the planets might

Orbit Comparison
between TRAPPIST-1 planets, Galilean
moons of Jupiter, and inner solar system
5
million km

Galilean
moons
Io
Europa
Ganymede
Callisto

TRAPPIST-1
planets
bc

g
h

100
million km
Mercury
Venus

Inner
solar system

Earth

p GALILEAN PLANETS All of the seven exoplanets discovered around
TRAPPIST-1 orbit much closer to their star than Mercury does to the
Sun, as shown here in this comparison of their orbits with those of the
Galilean moons of Jupiter and the planets of the inner solar system. But
because TRAPPIST-1 is far fainter than the Sun, the worlds are exposed
to similar levels of irradiation as Venus, Earth, and Mars.

Sky and Telescope - June 2017

Table of Contents for the Digital Edition of Sky and Telescope - June 2017