AST 251
Life on Other Worlds
Dr. Michael Reid
University of Toronto
ast251@astro.utoronto.ca
Credit: NASA/SETI/JPL
How Do We Find
Habitable Planets?
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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We have made an attempt to
define “life” and establish its
fundamental biochemistry.
Now it’s time to investigate
where in the cosmos life can
survive.
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We refer to planets orbiting stars
other than the Sun as extrasolar
planets or exoplanets.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA Ames/JPL-Caltech/T. Pyle
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You might imagine that we
would find exoplanets by
photographing them, the
same way we do with other
astronomical objects.
This technique is known as
direct imaging.
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Direct imaging is very difficult for the same
reason it is difficult to photograph a firefly
sitting on a car headlight.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: Flickr user mikelietz
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messy stuff left over
when we remove as
much of the star’s
light from the image
as possible
exoplanet
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
The Gemini
Planet Imager
on the Gemini
South telescope
in Chile imaged
the star 51
Eridanus and
found a new
exoplanet (‘b’) in
2014
(McIntosh et al., Science, 2015)
Credit: Gemini Observatory and J.
Rameau (UdeM) and C. Marois NRC
Herzberg
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Planets
orbiting the
star HR 8799.
This is typical
of the level of
detail we can
see with
direct
imaging
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The planets we can
currently see using
direct imaging are
usually young, hot
Jovian planets—not
likely homes for life.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credits: Danielle Futselaar & Franck Marchis, SETI
Institute
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There are several other
successful exoplanet detection
methods, including:
transits
radial velocity/Doppler method
astrometry
pulsar timing
transit timing variation
microlensing
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A transit occurs when one
astronomical object passes
in front of another from our
point of view.
star
a transiting planet
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A transit of the Moon across the Sun,
as seen by the STEREO spacecraft
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA/STEREO
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We can’t actually see
exoplanets transiting because
we don’t have powerful enough
telescopes to image the
surfaces of distant stars.
However, we can spot transits
in a star’s light curve.
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star’s brightness
The light curve of
a star is a graph
of the star’s
brightness
versus time
time
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star’s brightness
?
time
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Here, ingress and
egress are too
quick for you to
see the slope (but
you could see
them if you could
zoom the time
axis).
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA
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star’s brightness
1
Ingress and
egress take time,
so the light curve
is sloped.
0.9999
time
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Rstar
star’s brightness
Rplanet
1
Depth
0.99
time
cross−sectional area of planet
𝐃𝐞𝐩𝐭𝐡 =
cross−sectional area of star
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Rstar
star’s brightness
Rplanet
1
Depth
0.99
time
cross−sectional area of planet
𝐃𝐞𝐩𝐭𝐡 =
cross−sectional area of star
𝛑𝐑𝟐𝐩𝐥𝐚𝐧𝐞𝐭
𝐃𝐞𝐩𝐭𝐡 =
𝛑𝐑𝟐𝐬𝐭𝐚𝐫
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Rstar
star’s brightness
Rplanet
1
Depth
0.99
time
cross−sectional area of planet
𝐃𝐞𝐩𝐭𝐡 =
cross−sectional area of star
𝛑𝐑𝟐𝐩𝐥𝐚𝐧𝐞𝐭
𝐃𝐞𝐩𝐭𝐡 =
𝛑𝐑𝟐𝐬𝐭𝐚𝐫
𝐑 𝐩𝐥𝐚𝐧𝐞𝐭 = 𝐑 𝐬𝐭𝐚𝐫 𝐃𝐞𝐩𝐭𝐡
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Measuring transit depths allows us to
measure the sizes of each planet in an
exoplanetary system, even though we
can’t see the planets themselves.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA/JPL-Caltech
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Real transit light
curves have
some amount of
“noise” in the
data originating
from elements of
the telescope,
detector, etc.
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In real situations, it’s often difficult to
see the transit among all the “noise”.
(Zhou et al., AJ, 2017)
(Muirhead et al., ApJ, 2015)
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If we only see one transit,
we cannot be sure it is
caused by an exoplanet. It
could be caused by an
asteroid in our own solar
system, or a sunspot.
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA/SDO
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To confirm that a transit is caused by a
planet, we look for multiple transits
repeating at regular intervals.
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The time it takes a planet to
orbit its star is called the
orbital period.
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If we know the orbital period
of a planet, we can use
Kepler’s Laws of planetary
motion to calculate how far
the planet is from the star—
it’s orbital semi-major axis.
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The semi-major axis is
crucial to determining
habitability, as it tells
us whether the planet
is likely to be too hot,
too cold, or just right
for life.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: Carl Sagan Institute
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German
astronomer
Johannes Kepler
(1571-1630) worked
out the three laws
of planetary.
Kepler’s laws are
actually just
consequences of
Newton’s Law of
Gravity, which
hadn’t been
discovered in
Kepler’s time.
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Kepler’s First Law
All planets orbit their parent
stars in elliptical orbits with
the star* at one focus.
*This
is technically not correct—the center of mass of the
system is at one focus. However, in the general case that the
star is much more massive than any of its planets, the
difference is small. We will return to this point.
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focus
semi-minor axis
center
semi-major axis, a
c
focus
This ellipse has eccentricity = c/a = 0.6
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center
radius
This ellipse has eccentricity = 0 (i.e. it’s a circle)
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Orbits with different eccentricities
(thicker line  higher e)
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: YouTube user Ms. Edge
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Kepler’s Second Law*
As they orbit, planets sweep
out equal areas in equal
times.
*This is the one that confuses everyone. It’s really a statement of the
conservation of angular momentum. That language wasn’t accessible to
Kepler.
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Snapshots of an orbiting planet
spaced at equal times—the planet
moves faster closer to the star.
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Kepler’s Third Law
The square of a planet’s
orbital period is proportional
to the cube of its orbital
semi-major axis.
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2a
𝟐
𝟒𝛑
𝐏𝟐 =
𝐚𝟑
𝐆(𝐌𝐩𝐥𝐚𝐧𝐞𝐭 + 𝐌𝐬𝐭𝐚𝐫 )
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If we assume the mass of the
planet is much less than the
mass of the star, we can use
Kepler’s Third Law to work out
the planet’s orbital semimajor
axis from its orbital period:
𝟐
𝟒𝛑
𝐏𝟐 =
𝐚𝟑
𝐆(𝐌𝐩𝐥𝐚𝐧𝐞𝐭 + 𝐌𝐬𝐭𝐚𝐫 )
𝟒𝛑𝟐 𝟑
𝐢𝐟 𝐌𝐬𝐭𝐚𝐫 ≫ 𝐌𝐩𝐥𝐚𝐧𝐞𝐭
≅
𝐚
𝐆𝐌𝐬𝐭𝐚𝐫
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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𝟐
𝟒𝛑
𝐏𝟐 =
𝐚𝟑
𝐆(𝐌𝐩𝐥𝐚𝐧𝐞𝐭 + 𝐌𝐬𝐭𝐚𝐫 )
𝟐
𝟒𝛑
𝐏𝟐 ≅
𝐚𝟑
𝐆𝐌𝐬𝐭𝐚𝐫
𝐆𝐌𝐬𝐭𝐚𝐫 𝟐
𝟑
𝐏
≅
𝐚
𝟒𝛑𝟐
𝐆𝐌𝐬𝐭𝐚𝐫 𝟐
𝒂≅
𝐏
𝟐
𝟒𝛑
𝐛𝐞𝐜𝐚𝐮𝐬𝐞 𝐌𝐬𝐭𝐚𝐫 >> 𝐌𝐩𝐥𝐚𝐧𝐞𝐭
𝟏/𝟑
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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Let’s say P = 2.1 years and Mstar = 2.0 M☉. To
find a, first convert everything to standard
units (m, s, kg, etc.)
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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Let’s say P = 2.1 years and Mstar = 2.0 M☉. To
find a, first convert everything to standard
units (m, s, kg, etc.)
Convert P to seconds:
P = (2.1 y)(365.24 days/y)(86400 s/day) = 6.627 x 108 s
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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Let’s say P = 2.1 years and Mstar = 2.0 M☉. To
find a, first convert everything to standard
units (m, s, kg, etc.)
Convert P to seconds:
P = (2.1 y)(365.24 days/y)(86400 s/day) = 6.627 x 108 s
Convert Mstar to kg:
Mstar = 2MSun = (2)(1.989 x 1030 kg) = 3.978 x 1030 kg
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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Let’s say P = 2.1 years and Mstar = 2.0 M☉. To
find a, first convert everything to standard
units (m, s, kg, etc.)
Convert P to seconds:
P = (2.1 y)(365.24 days/y)(86400 s/day) = 6.627 x 108 s
Convert Mstar to kg:
Mstar = 2MSun = (2)(1.989 x 1030 kg) = 3.978 x 1030 kg
(𝟔. 𝟔𝟕𝟒 × 𝟏𝟎−𝟏𝟏 𝐦𝟑 𝐤𝐠 −𝟏 𝐬−𝟐 )(𝟑. 𝟗𝟕𝟖 × 𝟏𝟎𝟑𝟎 𝐤𝐠)
𝐚≅
(𝟔. 𝟔𝟐𝟕 × 𝟏𝟎𝟖 𝐬)𝟐
𝟐
𝟒(𝟑. 𝟏𝟒𝟏𝟓𝟗)
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
𝟏/𝟑
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Let’s say P = 2.1 years and Mstar = 2.0 M☉. To
find a, first convert everything to standard
units (m, s, kg, etc.)
Convert P to seconds:
P = (2.1 y)(365.24 days/y)(86400 s/day) = 6.627 x 108 s
Convert Mstar to kg:
Mstar = 2MSun = (2)(1.989 x 1030 kg) = 3.978 x 1030 kg
(𝟔. 𝟔𝟕𝟒 × 𝟏𝟎−𝟏𝟏 𝐦𝟑 𝐤𝐠 −𝟏 𝐬−𝟐 )(𝟑. 𝟗𝟕𝟖 × 𝟏𝟎𝟑𝟎 𝐤𝐠)
𝐚≅
(𝟔. 𝟔𝟐𝟕 × 𝟏𝟎𝟖 𝐬)𝟐
𝟐
𝟒(𝟑. 𝟏𝟒𝟏𝟓𝟗)
𝟏/𝟑
𝐚 ≅ 𝟑. 𝟎𝟗 × 𝟏𝟎𝟏𝟏 𝐦 = 2.1 AU
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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1 AU = 1 Astronomical Unit
1 AU ≈ average distance between
Earth and the Sun
≈ 150 million km
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: Wikimedia Commons user Huritisho
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star’s brightness
1
time
If the cadence of your observations is
too low, the curve can be difficult to
interpret.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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star’s brightness
1
time
If the cadence of your observations is
too low, the curve can be difficult to
interpret.
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star’s brightness
Could there be a pair of missing transits?
1
time
If the cadence of your observations is
too low, the curve can be difficult to
interpret.
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Lower cadence can also cause you to
miss ingress and/or egress, which are
typically very fast.
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As in our solar system, most stars
seem to have multiple planets. How
many do you see here?
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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As in our solar system, most stars
seem to have multiple planets. How
many do you see here?
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Hands-on activity
Using plot.ly to plot and
examine light curves
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Planets themselves emit &
reflect light, which means
that there’s a secondary
eclipse when the planet
passes behind the star.
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star brightness
no eclipse
(we see all of the light
from the star and the
planet)
primary transit
(planet covers
star)
secondary eclipse
(star covers planet)
time
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It’s even possible to use
transit light curves to learn
about the surface
temperature of a planet, but
only rarely.
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Just before the secondary
eclipse, the warmer, brighter
day side of the planet points
toward Earth.
The cold hot sides are
turned equally toward
Earth.
To distant Earth
Only the cool night side faces
Earth during the primary transit.
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The varying amount of
light from the planet
makes the light curve
vary in brightnes, even
between transits.
(Demory et al., Nature, 2016)
To distant Earth
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA/JPL-Caltech/University of Cambridge
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hot spot on the
side of the
planet which
faces the
parent star
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Biases of the
Transit Method
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transit visible from Earth
transit NOT visible from Earth
The transit method can only
detect planets whose orbits are
edge-on to our line of sight.
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Easier to detect
Harder to detect
Planets with larger radii are
easier to detect.
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Jupiter size
Earth size
The transit method has difficulty
finding Earth-sized and smaller planets
with current telescopes because
smaller planets produce shallower
transits.
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Results of the
Transit Method
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To date, the transit method
has been by far the most
successful planet-finding
method.
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Exoplanets by discovery year and method
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Much of the success of this
method was due to the
Kepler space telescope.
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Credit: NASA/Ames Research Center/W. Stenzel/D. Rutter
Kepler was operational from 2009-2018.
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Kepler has expanded the
range of known types of
planets beyond those
known from our own solar
system.
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“gas giants”
terrestrial planets
ice giants
jovians
Planets to scale; distances not to scale.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA
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Terrestrial planets: mainly
rock with thin atmospheres
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA/JPL
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Jovian planets: liquid hydrogen
with thick gaseous atmospheres.
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Ice giants/Neptunian: slushy
liquid interiors with thick
gaseous atmospheres
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Exoplanets come in types not
present in our solar system:
hot Jupiters
super-Jupiters
super-Earths
mini-Neptunes
lava worlds
ocean worlds
…
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA/Kepler
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA Ames/W. Stenzel
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA/Kepler
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There seems to
be a real lack of
planets in the
range around
1.75 Earth radii.
What could be the
cause?
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA/Kepler
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: NASA/Kepler
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Using the method of transit
spectroscopy, it is
sometimes possible to
measure the chemical
composition of exoplanet’s
atmosphere.
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Continuous spectrum
Every atom and
molecule absorbs a
characteristic set of
colours of light.
If we measure
transits in light of
many different
colours, we can spot
deeper transits
where particular
chemicals are
absorbing more light.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto Credit: astronoo.com
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The transit depth will be
shallower at a wavelength
(colour) where no chemical
is absorbing light from the
star (so only the solid body
of the planet blocks light).
Credit: NASA's Goddard Space Flight Center,
Additional animations courtesy ESA/Hubble
The transit depth will be
deeper at a wavelength
(colour) where a particular
chemical in the
atmosphere is absorbing
more light.
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AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA's Goddard Space Flight Center,
Additional animations courtesy ESA/Hubble
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Transmission spectrum of the
atmosphere of exoplanet WASP 19b.
AST 251: Life on Other Worlds | Dr. M. Reid | University of Toronto
Credit: NASA's Goddard Space Flight Center,
Additional animations courtesy ESA/Hubble
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Credit: NASA/Ames Research Center/Wendy Stenzel and The University of Texas at Austin/Andrew Vanderburg
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Exomoons
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As we have already seen,
moons of giant planets may
be the most numerous
habitable environments in
the cosmos.
Can we detect moons of
exoplanets—exomoons?
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star’s brightness
1
0.9999
time
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A planet with a
moon will wobble
as it orbits the
star. The exact
timing of
consecutive
transits will vary.
(Kipping, 2014, arXiv:1405.1455)
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In October 2018,
the first evidence
of an exomoon
emerged.
(Teachey & Kipping, Science, 2018)
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Credit: Dan Durda
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Questions?
Meet me outside the classroom or
visit me during office hours.
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