Transcript Chapter 3: the Sun - University of Waterloo

Extrasolar planets
Detection methods
1.
2.
3.
4.
5.
6.
7.
Pulsar timing
Astrometric wobble
Radial velocities
Gravitational lensing
Transits
Dust disks
Direct detection
1. Pulsar timing
•
•
Pulsars are rapidly rotating neutron stars, with extremely
regular periods
Anomalies in these periods indicate the gravitational influence of
a companion.
Astrometric wobble
•
•
Changes in proper motion are so
small that the best current
equipment cannot produce reliable
enough measurements.
This method requires that the
planets' orbits be nearly
perpendicular to our line of sight,
and so planets detected by it
could not be confirmed by other
methods.
3. Radial motions: the Doppler shift
Recall the Doppler shift of the wavelength of light due to the
velocity of the source:
vr obs  rest 


c
rest
rest
Spectroscopic binaries
The absorption lines are
redshifted or blueshifted
as the star moves in its
orbit
Spectroscopic binaries: circular orbits
• The radial velocities are a sinusoidal function of time. The minimum and
maximum velocities (about the centre of mass velocity) are given by
v1max
 v1 sin i
r
v2max
r  v2 sin i
• Where i is the angle of inclination.
Radial velocities: mass measurement
•
If the star can be
accurately classified (i.e.
with a good spectral
classification and a parallax
distance) we can determine
its mass independently of
the orbit.
1/ 3
max
2/3  P 
mPlanet sin i  vStar mStar 

 2G 
Radial velocities: mass measurement
1/ 3
max
2/3  P 
mPlanet sin i  vStar mStar 

 2G 
E.g. the star HD73256:
From Hipparcos data (and detailed
stellar modelling) we know
Mstar~1.05 Msun
• From the light curve we measure
P=2.54858 days and vmax=269.8 m/s.
• Sinusoidal shape means e~0
What is the mass of the planet, and the size of the orbit?
Radial velocities: difficulty
mPlanet sin i  1.86M Jupiter
a  0.037 A.U .
The maximum velocity shift is only ~270 m/s. The Doppler shift is therefore:



v
 9 10 7
c
which is very small. For example the Ha line is redshifted by only 0.00059 nm!
The spectral resolution must therefore be very high. Detecting smaller planets,
farther away from the star, is an even more difficult task.
Break
Gravitational microlensing
• This effect occurs when the gravitational field of a planet and its
parent star act to magnify the light of a distant background star
• The key advantage of gravitational microlensing is that it allows
low mass (i.e. Earth-mass) planets to be detected using available
technology.
• A notable disadvantage is that the lensing cannot be repeated
because the chance alignment never occurs again.
Transits
• Detects a planet's shadow
when it transits in front of
its host star.
• Can be used to measure the
radius of a planet.
Transits
• Imagine viewing the Earth-Sun system from a distant star. By
how much will the Sun fade during a transit of the Earth? How
about during a transit of Jupiter?
Circumstellar disks
Young main sequence stars often still have disks, even after the
molecular cloud has been dispersed.
Infrared-emitting dust disk around
b-Pic. The central star has been
subtracted.
The dust disk around Vega. At least
one large planet is known to exist
within this disk.
Circumstellar Disks
•
Orbiting planets can clear gaps in the dust disk
 This leads to a loss of orbital energy, so the planets “migrate”
inward
Direct detection
• Infrared image of the
star GQ Lupi orbited by a
massive, young (therefore
warm) planet at a distance
of approximately 20 times
the distance between
Jupiter and our Sun.
• 2005 image of 2M1207 (blue)
and its planetary companion, one
of the first exoplanets to be
directly imaged
7. Direct Detection
The albedo of the Earth is about AV=0.4. How bright is it in visible
(reflected) light, relative to the Sun? How do they compare at
infrared wavelengths, where Earth emits thermal radiation?
A picture of Earth,
from the surface of
Mars, just before
sunrise.
Atmospheres
HD 209458b:
• the first transiting planet discovered
• the first extrasolar planet known to
have an atmosphere:
 evaporating hydrogen
 contains oxygen and carbon.
• Recent Spitzer spectroscopy reveals:
 Less water vapour than expected
 Silicate dust clouds
Artist’s conception
Extrasolar planet searches
As of Feb 2007, 217 planets have been detected outside our
solar syste.
See http://exoplanets.org/
Most of these have a<1 AU and masses >MJupiter
Extrasolar planet searches
• Orbits tend to be quite eccentric
Future missions
Keck Interferometer
Large Binocular Telescope Interferometer
SIM PlanetQuest
Kepler
Terrestrial Planet Finders
19
Space Interferometry mission
http://planetquest.jpl.nasa.gov/SIM/sim_index.cfm
• Will search for terrestrial planets around the nearest ~250
stars, with astrometry accurate to 1 mas.
Kepler
• http://www.kepler.arc.nasa.gov/
• Will search large numbers of stars for Earth-sized terrestrial
planets using the transit method.
• sensitivity limits of radial velocity
surveys, astrometric surveys,
microlensing surveys, and spacebased transit techniques.
• The shaded areas show the
expected progress towards the
detection of Earth-like planets by
2006 and 2010.
• The filled circles indicate the
planets found by radial velocity
surveys (blue), transit surveys
(red), and microlensing surveys
(yellow).
• The discovered extrasolar
planets shown in this plot
represent the reported findings
up until 31 August 2004.