Download What did we learn from transiting planets?

What did we learn from transiting
exoplanets?
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Introduction to exoplanets
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Main questions, unsolved problems
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Transiting exoplanets
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Looking back ...
The idea of exoplanets and extraterrestrial life has been around
for 2300+ years
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Aristarchus of Samos (310—230 BC): heliocentric view, stars are
suns
Epicurus (341–270 BC): suggested that the universe is filled with
other worlds where extraterrestrial life is possible.
Giordano Bruno (1584): “There are innumerable suns and an
infinite number of planets which circle around their suns as our
seven planets circle around our Sun.”
Christiaan Huygens (1698): “What a wonderful and amazing Scheme
have we here of the magnificent Vastness of the Universe! So many
Suns, so many Earths... “ “Planets that surround that prodigious number
of Suns […] must have their plants and animals, and their rational
creatures too, and those as great admirers, and as diligent observers of
the heavens as ourselves ... “
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Looking back ...
The idea of exoplanets and extraterrestrial life has been around
for 2300+ years, which is >100 generations.
The discovery of exoplanets happened during our generation.
Note characters in red, and be a careful observer of details.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
The first exoplanets
• Via radial velocity observations, using moderately precise spectrographs
• Initial aim: substellar objects, brown dwarfs, or provide RV standards for
the IAU. RV precision reached 10—20 m/s.
• Campbell, Walker, 1988: Gamma Cephei, P=2.7yr, 1.7MJ. Very cautious
announcement.
• Latham et al. 1989,
reported P = 84d, 11MJ
companion to HD114762.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Are there planets closer in?
Struve, 1952. Also on ADS.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
51 Peg
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Doppler shifts of the star
51 Pegasi indirectly
revealed a planet with
4.2-day orbital period.
(Mayor & Queloz 1995.)
The short period was a
real surprise!
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Probability of transits
For Earth, p = 1/200
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Transiting exoplanet discoveries
• One famous example:
HD209458.
• 8th magnitude solar type
star in Pegasus.
• RV variations detected by
the Geneva team (Mayor
et al) and by the
California team (Marcy et
al).
• Subsequent follow-up by
Henry et al, and by
Charbonneau et al.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
How did hot Jupiters get there?
• These planets orbit their host stars at every
few (Earth) days.
• Equilibrium temperatures are ~2000 K.
• They could have not formed at their
present position:
1) No massive planetary embrio formation
inside the ice-line (amount of metals in a
protoplanetary nebula is small).
2) No enough material to sweep up to form
a massive planet.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Main questions regarding exoplanets
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How do planets (and other substellar objects )
form?
How do planets evolve? Evolution of their orbits
and their physical characterisics.
What are the physical characteristics of
exoplanets, and how do these relate to the
properties of their stars?
What is the occurrence rate of planets, moons,
including Earth-like rocky planets?
What is the frequency of life outside Earth?
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Exoplanet discovery
methods
• Timing (e.g pulsars)
• Reflex motion of the
host star (RV)
• Transits
• Microlensing
Wolszczan 1994
Wright et al. 2010
• Direct imaging
Marois et al. 2008
Department of Astrophysical Sciences --Gaudi et al 2008, OGLE-2006-BLG-109
Gáspár Bakos
Princeton University
PSR1257+12
•
The following two slides (like many others) were hidden in the
original presentation, as delivered on June 30, due to time
constraints. Nevertheless, in order to address criticism by Prof.
Kulkarni, they are shown in this PDF printout.
•
Note1: while these pulsar planets are indeed reach
super-Earth masses, little is known about them, such as radii,
temperatures, composition, etc, thus I maintain the view that it
is important to search for low mass/radius planets around
stars.
Note2: my talk also misses microlensing results, again, on
purpose, due to lack of time, and mismatch of context. This is
not a general overview of exoplanets.
•
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
PSR1257+12
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Rotation rate: 6.22 milliseconds
(9,650 rpm!)
Original discovery: two planets.
Minimum masses 2.8 and 3.4 earth
masses, distances from the pulsar
are 0.47 AU and 0.36 AU, almost
circular orbits with periods of 98.2
and 66.6 days
Arecibo, Puerto Rico
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
PSR1257+12
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One more planet found in 1994
(0.02 Earth mass = Moon!)
And another in 2002 (0.0004 Earth
mass, which is a Ceres-mass
body!)
Formation of planets probably after
the explosion.
Only two other pulsars are known
to harbor planets (PSR B1620-26
b, PSR B1719-14 b)
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Schematics of an exoplanet transit
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
In the meantime … the radial velocity of the
star also varies
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
1. Planetary masses, radii
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Inflated planets
Inflated with respect to
theoretical model
predictions.
Explanations include:
• Eccentricity tides
• Obliquity tides
• High atmospheric
opacity
• Thermal tides
• Ohmic dissipation of
electrical currents
• … etc.
Howard 2013, Science
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
On the migration of hot Jupiters
→ Hot Jupiters must have migrated in.
How?
• 1. Slow (called Type-II) migration?
• 2. Gravitational planet-planet scattering.
• 3. Kozai cycles: interaction due to an outer,
massive companion (such as a stellar
binary companion).
• How could we decide which mechanism is
the typical (for a given system, or in
general)?
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
How can we distinguish between these
formation and migration scenarios?
• → through the inclination between the stellar spin axis and the
planetary orbital normal.
• Type-II migration → would dampen mutual inclinations.
• Gravitational planet-planet scattering → broad range of inclinations.
• Kozai cycles: interaction due to an outer, massive companion (such
as a stellar binary companion) → very broad range of inclinations.
• We think that the close alignment of planetary orbital normals and Sun
spin axis follows from the SS formation from a spinning
protostellar/planetary nebula.
• Gross misalignments can hint on different formation and evolution
scenarios.
• How can we measure the inclinations for exoplanetary systems?
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
The Rossiter-McLaughlin (RM) effect
• λ = 30 (XO-3b)
• λ = 165 (HAT-P-6b)
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Hot stars with hot Jupiters have high obliquities
Albrecht et al, 2012
• Teff >6250 K → preferentially misaligned
• Retrograde → M < 3MJ?
• M>8MJ → misaligned?
•Somehow related to tidal dissipation timescales?
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
How can we distinguish between these
formation and migration scenarios?
• → through finding perturbing companions, aka “cold friends of hot
Jupiters” that may have been responsible for the migration of these
planets.
• long term radial velocity monitoring
• high resolution imaging.
• >10% of hot jupiters have such “cold friends”
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Secondary eclipses = occultations
Occultation: when the planet goes behind the star.
• Small drop in overall flux. Reflected light + thermal emission.
•
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Occultations
• HD189733b: a famous transiting extrasolar planet.
• P = 2.2d , M = 1.1 MJ, R = 1.1 RJ around a K dwarf star. A
classical hot Jupiter.
• Star has a small and faint M dwarf companion.
• Curiously, it is right next (towards) the Dumbbell nebula.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
3. Occultations → brightness temperatures
• The contrast (flux ratio)
between the planet and the
star does depend on the
wavelength, even in the IR
domain.
• Broadly speaking, the
contrast increases as we
move to longer wavelength.
• Primary reason: the RJ tail is
an approximation. In reality,
these are ~Planck-curves.
Further departures are due
to the planetary atmosphere,
clouds, etc.
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
4. Thermal brightness maps
HD189733b is tidally
locked, i.e. showing the
same face towards the
star.
• It has a hot spot, not
exactly facing the star, but
somewhat 'behind'. This is
due to atmospheric
circulation on the planet.
•
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
5. Occultation in visible light → albedo
HAT-P-7b Kepler
observations (in
visible light, close to
the peak intensity of
the host star) reveal
the occultation due to
the planet.
• This is the reflected
light from the planet!
(Not infrared thermal
emission).
•
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
TrES-2b: the darkest 'world'
• Kepler observations in visible
light reveal the occultation due
to the planet.
• Depth is 5 parts-per-million
(ppm). 3.5-sigma result.
Kipping & Spiegel 2012
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Cloud maps of Kepler-7b
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Transmission spectroscopy
Starlight shining
through the planetary
atmosphere.
• Note the discovery of
Venus' atmosphere
during the 1769
transit!
• Planet atmosphere
absorbs light in
particular
wavelengths,
depending on its
composition!
•
From Michael Perryman
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Transmission spectroscopy
• Equivalent to the
planet appearing
'larger' during transit in
selected bandpasses
(i.e. transit being
deeper in certain
wavelengths).
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Composition of atmospheres
• Transmission spectrum for HD189733b
• Combination of many observations.
• Solid lines show theoretical models. Observations are indicated by
points and error-bars.
• Various elements (Na) and molecules (water, methane) have been
detected.
• Detections and their significance have been subject to intense debate.
See e.g. 2014ApJ...784..133S
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
High resolution ground-based spectroscopy of
(transiting) exoplanets

Molecular absorption in exoplanet atmospheres, both in
night side (transmission) and dayside (thermal emission)
spectra.
Other systems inculde:
HD189733b orbital motion in CO
absorption, de Kok et al 2013
Tau Boo b, 51 Peg b: CO
HD209458b orbital motion in CO transmission,
Snellen et al 2009
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Planet occurrence rates
• df(R)/dlogR = 3 * R-2
• HJ are extremely rare (1% of
stars have a HJ)
• Small and long period planets
are frequent
• df(M)/dlogM ~ M-0.5
• Giant planets: f ~ metal content
and mass of host star
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
eta Earth
• Petigura et al. 2013, ApJ, based on Kepler: “We find that
15.1^{+1.8}_{-2.7}% of solar type stars—roughly one in six—has a 1-2 R_E
planet with P = 5-50 days. “
• Petigura et al. 2013, PNAS, based on Kepler: “Extrapolating, one finds
5.7^{+1.7}_{-2.2}% of Sun-like stars harbor an Earth-size planet with orbital
periods of 200-400 d. “
• Marcy, 2014, PNAS: “Defining habitable zones remains difficult”
• Foreman-Mackey et al. 2014, "We find that the rate density of Earth analogs
is about 0.02 (per star per natural logarithmic bin in period and radius) with
large uncertainty. This number is much smaller than previous estimates
made with the same data but stronger assumptions. “
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Circumbinary planets: Kepler-16b
• The first Tatooine
(circumbinary planet).
• Star Wars!
• Science is stranger
and weirder than
fiction (can even
think of).
• P=229d
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Kepler-47b,c
www.youtube.com/watch?v=dLGhkwfyb70
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
Current census: 1800 planets
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
The Kepler orrery
• The Kepler orrery by Dan Fabrycky (multiple transiting Kepler systems).
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
• www.youtube.com/watch?v=gnZVvYm6KKM
The TESS project
• TESS: Transiting Exoplanet
Survey Satellite
• Observe 500K bright stars at
0.5 mmag precision (9min),
27d timespan, 1min
cadence.
• All sky survey
• Expected to find ~2700 TEPs,
with ~300 SuperEarths
• Targets much brighter than
Kepler and Corot
• TESS + VLT/GMT/TMT/ELT,
TESS + JWST: where,
how, when to look
• VLT/HARPS-N/E-ELT: radial
velocity, mass
determination, excluding
blend scenarios
• JWST: IR transmission
spectroscopy, CO2, H2O
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
JWST
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University
What will we learn from transiting planets?
Further physical characteristics, such as
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composition of atmospheres, including clouds
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spin of planets (rate, direction)
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volcanism
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rings
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oblateness of planets
• Formation and migration of planets (in progress) via e.g.
measuring the orbital tilts for more planets
• Frequency and nature of exomoons
• Abundance of life in the Universe
➔
Gáspár Bakos
Department of Astrophysical Sciences --- Princeton University