Download How Common Are Planets Around Other Stars? Transiting

How Common Are Planets Around Other Stars?
Transiting Exoplanets
Kailash C. Sahu
Space Tel. Sci. Institute
Earth as viewed by Voyager
Zodiacal cloud
"Pale blue dot"
Look again at
that dot. That's
here. That's
home. That's us.
On it everyone
you love,
everyone you
know, everyone
you ever heard
of, every human
being who ever
was, lived out
their lives....--on
a mote of dust
suspended in a
sunbeam.
-Carl Sagan
(Pale Blue Dot)
Sky at Cerro
Tololo, Chile
Is "Pale blue dot" unique?
Are there other planets out there around other stars?
Extrasolar Planet Discoveries
Search for planets around other
stars began more than100 years ago.
The first extrasolar planet around a
normal star, 51 Peg, was discovered
in 1995 through RV technique. This
has a period of 4.2 days.
>300 exoplanets discovered so far,
mostly within ~500 light years.
They are Jupiter or Neptune-like
planets, include multiple planet
systems, and planets in
~habitable zones.
They include ~50 transiting planets.
Extrasolar Planet Discoveries
Transits
•Transiting planets are
ideal for studying the
properties of atmospheres.
•Sodium and hydrogen
have been detected in the
atmosphere of one such
planet using HST.
•Spitzer observations have
recently shown silicate and
water emission.
Transit by the planet
•SPITZER has directly detected the transit by the planet, and
has led to “temperature mapping” of the planet.
1.01
Relative Flux
1.00
0.99
0.98
0.97
a
Relative Flux
1.003
1.002
1.001
1.000
b
0.999
-0.1
0.0
0.1
0.2
0.3
Orbital Phase
0.4
0.5
0.6
First Visible Light Image of
an Extrasolar Planet by Hubble
✦Discovered by Kalas et al. in 2008 using Hubble.
✦Fomalhaut b is at ~119 astronomical units (AU) from the star
✦25 light years from Earth
What about Earth-like Planets?
• Microlensing is currently the
only technique capable of
detecting earth-mass planets at
distances of several AU, and at
large distances from us.
Microlensing
Red:lens
Green: source
Blue: images
Chief Scientist
for Australia
Penny Sackett
PLANET Collaboration
(Probing Lensing Anomalies NETwork)
Founded in 1995 by
Penny Sackett & Kailash Sahu
If every star has a Jupiter, 1/7 should should show planetary
signal. Se we needed to monitor at least 7 events per year.
So far, we have monitored > 200 events, including many
binaries, and discovered one planet.
OGLE-2005-BLG-390
PLANET Observations:
Extra peak: 9 August, 2005
Additional obs MOA/OGLE
5.5 Earth-mass planet at
2.3 AU from a star of 0.25
solar mass.
Text
Text
Text
Two more probable
detections by other groups.
Microlensing observations suggest that Earth-mass planets
must be more abundant than Jovian planets at > 1 AU.
OGLE-2005-BLG-390
PLANET Observations:
Extra peak: 9 August, 2005
Additional obs MOA/OGLE
5.5 Earth-mass planet at
2.3 AU from a star of 0.25
solar mass.
Text
Text
Text
Two more probable
detections by other groups.
Microlensing observations suggest that Earth-mass planets
must be more abundant than Jovian planets.
The 5 earth-mass (m sin i) planet around GL581, with a period
of ~13 days, supports this suggestion.
What do we know so far........
• More than 300 extrasolar planets have been detected.
• The planet detections have been mostly confined to 500 light years.
• They suggest that about 6% of stars have Jupiter-like planets.
• Earth-like planets are more common.
An analogy.....
Zodiacal cloud
"Pale blue dot"
•There are 100 billion stars in
"Solar
neighborhood"
the Galaxy, stretched over
100,000 light years.
•Are planets equally common in the entire Galaxy?
•To explore this question, we should look at a very different
part of the Galaxy.
• The technique: transits
Characteristic light
curve shape
Same in V and I
Must repeat
No variation outside
transit
Depth must be
consistent with
planet
Probability of detection is small.
Probability of
planet detection
*Approximate numbers
SWEEPS Project
Location of
SWEEPS field
Ideal for
studying our
key questions
Monitored this
field
continuously
for 7 days
SWEEPS Project
Sagittarius Window
Eclipsing Extrasolar
Planet Search
202”x202”
Continuous monitoring
for 7 days
180,000 stars to V ~ 27.
245,000 stars to V ~ 30
265 I images
254 V images
339 sec Int. time
No transits missed
Deepest Galactic field
FINDING PLANETARY TRANSITS:
An analogy with
MIRACLE IN THE 34th STREET (Baltimore)
We discovered 16
Planet Candidates
(the largest
number from a
single set of
observations).
Example light curves
in Blue and Red
Black: Model fit
Includes two RVconfirmed planets.
This program can
detect Jupiter-like
planets around
small stars (M~0.44
solar mass).
16 planetary
candidates
(circled)
We also
detected 165
eclipsing
binaries.
125 of them
show
ellipsoidal
variations
caused by
close binaries.
Properties of the Planet Candidates
• These are the farthest planets known to date
(26,000 light years away).
• Host masses are 0.44 to 1.24 M⨀ (lowest
mass host stars so far).
• 5 planets have periods of < 1.0 d, shorter
than the shortest period previously known.
(Ultra-Short-Period Planets, or USPPs).
ARE THEY REALLY PLANETS?
False Positive Contributions
• Blending
• Grazing eclipses
• Low-mass stars
Blending
of an eclipsing binary with a
• Blending
brighter star can mimic planetary signal.
used the binary statistics to estimate
• We
that the blending fraction is less than one.
Grazing Eclipses
Estimated contamination ~ 1.4 (conservative since some
grazing incidences are clearly recognizable as such).
Low-Mass Stars
Star
BD
Hot Planet
Jupiter
•
1-100 MJup. objects have similar radii, so it is difficult to rule
out brown-dwarfs or low-mass stars from transit signals.
One out of six may be low-mass stars.
•
In summary: at least ~half of the candidates must be
genuine planets.
•
RV measurements only can provide unambiguous
confirmation.
Radial Velocity Observations
• 4 nights of observations with VLT + UVES
• June 21-25, 2004. Long nights, clear weather, superb seeing
• We could observe 2 that were bright and isolated enough
Radial Velocity observations
with 8-meter VLT
SWEEPS-04
V ~18.8
brightest candidate
< 3.8 MJup
SWEEPS-11
V ~19.6
Planet detected at
9.7 MJup
All planets
with P < 12
days
Blue: RV planets
Green: Transits
Red: SWEEPS
SWEEPs sample
extends to 0.45M,
leading to USPPs.
Closer-in planets might be evaporatively destroyed, or
planets can migrate to and survive in close-in orbits only
around such old and low-mass stars.
Artist’s Impression
USPPs were not
detected before
since not enough
low-mass stars with
suitable metallicity
were monitored.
Frequency of Planets
Taking into account:
(i) the geometric probability, and (ii) the detection
efficiency, we estimate that ~0.5% of the bulge
stars are orbited by “hot Jupiters”.
From (i) Planet frequency from the local sample,
(ii) the metallicity~planet-frequency dependence,
(iii) metallicity distribution of bulge stars,
we predict 0.5% to have such planets.
The Jovian planet frequency in the bulge is thus
similar to that of the solar neighborhood (5%).
Earth-like planets are expected to be more
common.
Key implications
•The planet frequency at 2 very
different parts of the Galaxy are
similar.
•This gives us some indication that
the planets may be equally
abundant in the entire Galaxy.
•There are 100 billion stars in the
Galaxy.
•So there may be 6 billion Jovian
planets, and an even larger number
of Earth-like planets in the Galaxy.
Beyond the Galaxy?
UDF is the deepest
image of the sky
This tiny region of the
sky contains ~3000
galaxies
Universe may contain
~100 billion galaxies
The bulge is similar to the most common type of galaxy,
“elliptical”. So it seems reasonable to expect that planets
may be equally common in the whole Universe.
Future Missions: I. Kepler Mission
NASA’s first mission
capable of finding
Earth-like planets
Launch date:
2009 March 5
• Kepler will continuously monitor 100,000 stars for 4 to 6
+ years
• Expected to find many Earth-like planets around Sunlike stars
Future Missions: II. James Webb Space Telescope
NASA’s next large telescope
6.5 m segmented primary
Launch: 2013
Atmospheric Transmission Spectrum
(4 hr) for HD209458-like Kepler source
using NIRSpec R~3000 grating.
Simulations by J. Valenti, STScI.
• JWST can detect atmospheres and possible biomarkers
from Earth-like planets.
SUMMARY
More than 300 exoplanets
have been discovered to
date, including the first
direct image of a planet.
But they are all confined to
the ~500 light years from
the Sun.
Planets are equally abundant in
the Galactic bulge, and around
low-mass stars. 6% of stars have
Jovian planets. Earth-mass
planets must be more common.
This project led to the
discovery of USPPs,
which occur only
around low-mass stars.
Our SWEEPS project discovered 16 Planets may be equally
exoplanet candidates, first
common in the whole
exoplanets discovered by HST. This Universe.
represents the farthest sample at
26,000 light years, and the largest
number of planets discovered from
a single set of observations.
Is life common in the Universe? Kepler and JWST may provide some insight.