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Investigating the structure of transiting
planets, from hot Jupiters to Kepler super
Earths
Jonathan Fortney
University of California, Santa Cruz
Thanks to: Neil Miller (UCSC) , Eric Lopez (UCSC)
Eliza Miller-Ricci Kempton (UCSC), Nadine Nettelmann (U. of Rostock)
J
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Transiting Planets,
Large and Small
 110 planets have now been
seen to transit their parent
stars
 99 “hot Jupiters”
 5 “hot Neptunes”
 6 “super Earths”
 Combination of planet
radius and mass yield density -> composition
Strong bias towards finding
mass/large planets on shortperiod orbits
July 2007
There is an incredibly diversity of worlds
 We can also characterize these planets, not just find them
Late 2006
The shear number of
discoveries opens up the
prospect of understanding
gas giants (Jupiter-like), ice
giants (Neptune-like) and
lower mass planets as
classes of astrophysical
objects
Charbonneau, et al., 2007
 There is considerable diversity amongst the known
transiting planets
 Radii for planets of similar masses differ by a factor of
two, which cannot happen for pure H/He objects
Our Gas Giant Prototypes: Jupiter and Saturn
5-25% Heavy Elements by Mass
Fortney, Baraffe, & Militzer (2010)
Our Ice Giant Prototypes: Uranus and Neptune
80-90% Heavy Elements by Mass
Fortney, Baraffe, & Militzer (2010)
At Gyr ages, ~1.3 RJ is the largest radius of a standard cooling model
Fortney et al. (2007)
Building a Model, II: Additional Interior Power
Miller, Fortney, & Jackson (2009)
1 MJ planet with a 10 ME core, at 0.05 AU from the Sun
Explaining Large Radii
An area of active
research!
Beyond Radius Inflation: What are We Trying to Learn?
•We’d like to understand
giant planets as a class of
astrophysical objects
•What are their unifying
properties?
There is an emerging population of planets with no radius anomaly
Miller & Fortney (2011), submitted
A strong correlation between star and planet abundances
Miller & Fortney (2011), submitted
See also, Guillot
et al. (2006)
A quasi-uniform super-solar enrichment above 0.5 MJ
[Fe/H]<0.0
0.0≤[Fe/H]<0.2
0.2≤[Fe/H]<0.4
Solar=0.014
Miller & Fortney (2011), submitted
Implications for Giant Planets
•Giant planets, as a class, are enriched in heavy elements
•Enriched compared to the Sun
•Enriched compared to their parent stars
•Enrichment is a strong inverse function of mass, but with an
apparent “floor” at high mass
•The heavy element mass of an inflated
planet could be estimated only from its
stellar metallicity
•With that in hand, its additional interior
power could be constrained
•Radius inflation mechanism can be
studied vs. orbital separation and planet
mass
•Massive planets and low-mass brown dwarfs should have structural
and atmospheric abundance differences
There is an incredibly diversity of worlds
 We can also characterize these planets, not just find them
Charbonneau et al. (2009)
GJ1214b: A “Super Earth” orbiting
a nearby bright M star
What is the Nature of the Planet’s Atmosphere and Interior?
•Mass-Radius leads to degenerate solutions:
•Mostly water with a small rocky core
•A “failed” giant planet core?
•Lower ice/rock ratio, with a H/He envelope
•A mini Neptune?
What is the cooling history and interior state of these two
kinds of models?
Water World Model
Mini Rocky Neptune Model
Boundary in P(Mbar)/T(K)
Water World Model
The Atmosphere is the Key to understanding the Interior
H2/He-dominated
atmospheres
Miller-Ricci & Fortney (2010)
Bean et al. (2010)
The Kepler Mission
• Monitoring 150,000 stars for 3.5+
years
• 20 months into the mission
• First 4 months is now public
• 1200+ transiting planet candidates
• d < 0.25 AU
Borucki et al. (2011) Analysis: 2-3 RE Most Common Size
Analysis of first 4 months of data---much more still to come
Borucki et al. (2011) Analysis: 2-3 RE Most Common Size
Kepler-11
•The most densely-packed
planetary system yet found
•5 planets within the orbit of
Mercury
•Masses obtained only from
Transit Timing Variations,
with no Stellar RV
•Relatively low density for all
planets implies thick H/He
atmospheres
Kepler-11: Picking out the Planets
Kepler-11: Lightcurves and Transit Times
Kepler-11: The Mass-Radius View
GJ 1214b
• Modeled as rock-iron cores with water or H/He envelopes
• Atmospheric escape with time is ignored
Atmospheric Gain and Loss
CoRoT-7b
Jackson et al. (2010)
• In the Kepler-11 system, significantly more massive
planets can be ruled out from stability
considerations, particularly for the inner 2 planets
Alibert et al. (2005)
Conclusions
• A batch of new discoveries show that “mini-Neptunes” may be a
common (the most common?) type of planet
• The processes that affect H2-dominated atmosphere gain/escape
should be investigated in much more detail
• The Kepler-11 system is a natural laboratory to study
atmospheric mass loss
•Planet types keep emerging that we have no analog for in the solar
system
• We can now begin to understand the structure of giant planets with
lower-irradiation transiting planets
• Kepler has already found a larger sample of these types of
planets, but follow-up observations for masses must be done