Discovery of a cool planet of
5.5 Earth masses through
gravitational microlensing
Vol 439|26 January 2006|doi:10.1038/nature04441
Contents
• Preview & Animation
• Introduction & Abstract
• Figure & Data
• Bayesian Analysis
• Further
Exoplanet
• Hanno Rein
OGLE-2005-BLG-390Lb
• Here we report the discovery of a 5.5+5.5
−2.7 𝑀𝐸 planetary companion at a
+0.21
separation of 2.6+1.5
AU
from
a
0.22
−0.6
−0.11 𝑀𝑆 M-dwarf star
• Our detection suggests that such cool, sub-Neptune-mass planets may
be more common than gas giant planets, as predicted by the core
accretion theory.
*Planet formation via core accretion
• The most commonly accepted mechanism for the formation of Jupiter-like planets.
• In this model a rocky core forms through the coagulation of planetesimals until it
is sufficiently massive to accrete a gaseous envelope.
• Initially this envelope is in hydrostatic equilibrium, with most of the luminosity
provided by the accreting planetesimals.
• Once the core reaches a critical mass, however, hydrostatic equilibrium is no
longer possible, and a phase of rapid gas accretion occurs.
Ken Rice's homepage
Features
• Light curve deviations with features lasting a few hours.
• Most sensitive to planets in Earth-to-Jupiter-like orbits with semi-major axes in
the range 1–5 AU.
• Restricted by the finite angular size of the source stars
• Detection of planets as small as 0.1𝑀𝐸
• High sampling rate: > 500 microlensing events
Figure
• 𝐴𝑚𝑎𝑥 = 3.0
• 𝑞 = 7.6 ± 0.7 ×
10−5
• 𝑑 = 1.610 ±
0.008𝑅𝐸
• 𝜒 2 = 562.26
• 650 data points, seven
lens parameters, and
12 flux normalization
parameters, for a total
of 631 degrees of
freedom.
Supplementary Data
• Angular radius 5.25 ± 0.75 𝜇 as
• Source radius 9.6 ± 1.3𝑅𝑆 if the source star is at a distance of 8.5 kpc
• 5,200 K giant, which corresponds to a G4 III spectral type
• Einstein ring radius 𝑅𝐸 (typically, 2 AU for a Galactic Bulge system)
• Linear limb darkening laws with Γ1 = 0.538 and Γ𝑅 = 0.626
• Four different binary lens modelling codes were used to confirm that
the model we present is the only acceptable model for the observed
light curve
Supplementary Data (Cont.)
projected planet–star separation
planet–star mass ratio
Bayesian Analysis
• 95% probability that the planetary host star is a main-sequence star,
• 4% probability that it is a white dwarf,
• <1% probability that it is a neutron star or black hole.
+1.5
• a 5.5+5.5
𝑀
planetary
companion
at
a
separation
of
2.6
−2.7 𝐸
−0.6 AU from a
0.22+0.21
−0.11 𝑀𝑆 M-dwarf star
• 𝐷𝐿 = 6.6 ± 1.0 kpc
• Temperature ~50K
Further
• The separation of 𝑑 = 1.61 is near the outer edge of the so-called
lensing zone, and the planet’s mass is about a factor of two above the
detection limit set by the finite size of the source star.
• Planets with 𝑞 > 10−3 and d < 1 are much easier to detect