http://www.skyandtelescope.com.au 39Orbit Comparison
between TRAPPIST-1 planets, Galilean
moons of Jupiter, and inner Solar SystemTRAPPIST-1
planets5
million km
Io
Europa
Ganymede
Callisto100
million kmEarthVenusMercury Inner
solar systemGalilean
moonsbc
de
fg
hFrom 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 ofthree.
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
originalthirdplanetdoesn’texist.)
Only six of those are firm detections, however. The
seventh planet, designated h, is iffy in its specs: The team
detected only a single transit for it, and astronomers prefer
to see three transits before calling something a candidate
planet.Expectthemtohaggleoverthisone.
But let’s assume for now that all seven exoplanets are
real. All their orbits would easily fit inside Mercury’s circuit
aroundtheSun.Theyearsoftheinnersixrangefrom1.5
to12Earthdays,withtheperiodofoutermosthbeing
between14and35days—givingitanorbitlessthan20%
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
isthattheexoplanetshaveresonantorbits: Their orbitalperiods are roughly integer ratios of one another, a set-up
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 thus their approximate
masses and densities. All are consistent with being rocky, the
team concluded in the February 23rd issue of 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
planet-forming disk, gas friction 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
experience huge tidal pulls, stretching and squeezing their
interiors and spurring heating and even volcanism, similar to
what we see on Jupiter’s Galilean moons.SGALILEAN 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.Time (Universal Time)Relative brightness00:58 01:26f01:55 02:24 02:530.970.980.991.00e cfSSTARLIGHT 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.LEAH TISCIONE /
S&T
, ESO / M. GILLON ET AL. (2)