An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30
Exoplanets: Planets Around Other Stars
Search for planets around other stars.
Current Search Techniques:
- Astrometric Wobble Method
- Doppler Wobble (RV) Method - most successful so far
- Planetary Transit Method
- Gravitational Microlensing Method
Extrasolar planetary systems:
- Many Jupiter-sized planets close to their parent stars.
Are we alone in the Universe?
The question of the existence of other planets beyond the solar system,
and whether any can harbor life is an old one in astronomy.
- Are there solar systems around other stars?
- Are such solar systems like ours or different?
- Are any of the planets like the Earth?
- Has life arisen on other planets?
- Has intelligent life arisen on other planets?
In science, we prefer questions we can answer quantitatively to idle
The scientific problem has become one of searches for:
- Solar systems in the process of formation.
- Evolved solar systems around other stars.
- Evidence of life on other planets.
- Evidence of technological, intelligent life on other planets (SETI).
Searches for Extrasolar Planets
There are two basic search strategies:
- Take pictures of planets orbiting other stars
- Observe the transits of planets across the disks of their parent
stars, which causes a characteristic drop in brightness.
- Orbital motions ("wobbling") of the star because of the planet's gravity.
- Gravitational microlensing of a background star by the planet.
Recall Newton's form of
Kepler's First Law of Planetary Motion:
Viewed from afar, the star will appear to wobble about
the center of mass of the star-planet system.
- Planets orbit on ellipses with the center of mass
at one focus.
- The star also orbits around the planet-star center of
mass, but much closer to the center of mass at
a slower orbita speed because of its
There are two manifestations of this motion: Astrometric Wobble
and Doppler Wobble.
Parent star wobbles back & forth on the sky as seen relative to
distant background stars.
- The star is so much more massive than any planets, it will
be close to the center of mass, and the size of the wobble
wil thus be very small.
- The reflex motion is best seen when we are looking down
on the plane of the orbit.
From 18 light years away, the astrometric wobble of the Sun is
This method has been tried, but with limited success so far. Future
high-precision astrometric satellites like SIM (Space Interferometer
Mission) and GAIA will have the precision required to measure
astrometric wobbles from planetary systems.
Another way is to use the Doppler
Effect to detect the orbital motions of the wobbling star.
Measuring the orbital motions provides an estimate of the unseen
planet's mass via Newton's
form of Kepler's Third Law of Planetary Motion.
- Star's spectral absorption lines shift towards the blue
when the wobble moves the star towards the Earth.
- Star's spectrum shifts towards the red when
the wobble moves the star away from the Earth.
The greater mass of the star put it close to the center-of-mass
of the star-planet system, and thus it has a very slow
Example: Orbital Speeds of the Sun & Jupiter
The challenge is to measure the Doppler shifts of the lines with
extremely high precision.
- Jupiter: 13 km/sec at 5.2 AU from the C-of-M
- Sun: 13 meters/sec at 0.0052 AU from the C-of-M
To convey some idea of the scale of the problem, most people can walk at
a speed of about 1 meter/sec, while a car moving 65 MPH is moving at a
speed of 29 meters/sec.
- The current state-of-the-art is 3 meters/sec
- New techniques can achieve <1 meter/sec!
Michel Mayor & Didier Queloz at Geneva Observatory observed a
periodic wobble in the star 51 Pegasi in 1995.
- Sun-like star about 40 light-years away in the
constellation of Pegasus
- Wobble was 56 meters/second, with a period of only 4.2
- This implied a planet with a mass of 0.5 Jupiters
orbiting at 0.05 AU !
This was the first planet found around a sun-like star using
the Doppler wobble method.
It was quickly followed by other discoveries by teams in
California, Texas, and Europe. RV searches are now the primary
way people search for exoplanets around nearby stars.
Advantages of the RV Method
So far, the RV method has been the most successful to date
The RV method is very sensitive to massive planets around relatively
- About 85% of all exoplanets known thus far have been found
using the RV method
- Sensitivity to planets increases with time (need to sample one or
two whole orbits to confirm)
- Method gives an immediate estimate of the minimum mass of the exoplanet,
allowing reasonable confirmation that it is a planet and not some kind
of weird binary star system.
If the orbital plane of an extrasolar planet is aligned with
the line of sight:
So far, 33 transiting planets are known, 5 of which were previously
discovered using RV techniques. Large-scale searches are underway
that are rapidly increasing this count.
- The planet will periodically cross ("transit") the face of
its parent star.
- The star dims by 1% or so during the transit.
- Requires precision photometry & lots of luck.
- Biased towards finding very close-in Jupiter-sized planets.
The Case of HD209458
The first confirmed transiting planet. HD209458 is a star with a
Jupiter-sized planet found originally via the RV method:
Using an orbit prediction from the Doppler work, astronomers observed
HD209458 and were able to see the planet transit its parent star.
- Sun-like star 158 light years away in Pegasus with a Mass of
1.06 MSun and Radius of 1.18 RSun.
- Planet has a mass of ~0.69 Jupiters
- Circular orbit with a semi-major axis of 0.045 AU and
a period of ~3.5 days
Advantages of Transits
Transits offer the only way we currently have to make a direct measurement
of the radii of exoplanets
The only way we have to probe the atmospheres of exoplanets
- Gives an estimate of the density
- Densities are important clues to the composition of the exoplanet
(gas giant, ice giant, rocky planet, etc.)
The latest application of the Transit Method from space holds out the
possibility of detecting Earth-mass planets. Recent missions are the
European COROT satellite and the upcoming US KEPLER mission.
- Absorption lines seen in the parent star's spectrum from the planet's
cooler atmosphere during transit gives us an idea of the atmosphere's
- Thermal Infrared emission from hot exoplanets, especially during the
secondary eclipse (when the exoplanet is eclipsed by its parent star)
has given us information on the atmosphere.
If two stars line up, one near and the other far, the light from
the background star passing around the foreground star will be
bent by the foreground star's gravity.
- If the line up is close, this results in a dramatic increase
in brightness of the background star by the foreground "lensing"
- The "microlensing event" can last anywhere from days to months.
If there is also a planet around the foreground lensing star, its
gravity will also produce a brief, intense amplification if it passes
close to the line of sight.
- Offers another way to find planets around other stars.
- One of the few ways to find planets around very distant stars.
To date, 8 planets have been found by gravitational microlensing,
6 by OSU's Microlensing Follow-Up Network
Advantages of Microlensing
Microlensing is superbly sensitive to planetary systems like our own
It offers one of the few ways to find planets around more distant stars
- It is not biased towards finding close-in Jupiters like
the RV or Transit methods
In principle, Microlensing may be the only way we currently have that
could detect Earth-mass planets from the ground. This is much cheaper
than expensive space missions, and can be done by networks of small
amateur and professional telescopes.
- Aren't just limited to nearby strs
- Could give a more fair census of planetary systems
OSU is a leading player in the Microlensing Planet Search effort, and
has organized the largest amateur and professional observing network,
the MicroFUN Collaboration.
Roster of New Planetary Systems
As of November 2007, all of these techniques have found more than
260 planets around more than 220 stars:
- Most are single giant planet systems
- 25 multiple-planet systems, the largest being the 5-planet system
around the star 55 Cancri.
- Most are Jupiter-sized or larger (up to 13 times Jupiter's mass),
with some recent detections getting into the Neptune range, and
a couple of tantalizing "Super Earths" down to the 5MEarth
- All planets found thus far orbit within ~5 AU of their parent star.
Strange New Worlds
None of the planetary systems found so far resembles our Solar System.
The biggest surprises:
- Finding many Jupiter-sized planets very close to their
- Some, called "Hot Jupiters", are on orbits smaller than
that of Mercury, and have periods less than 10 days!
- Many of the Jupiter-sized planets are on very elliptical
What is going on? This is a subject of much current research.
There are continuing searches for other planetary systems.
- Find systems more like our own Solar System
- Assess how common planetary systems are
This is considered one of the most important astronomical research
programs of the 21st Century.
- Space missions being planned to search for Earth-mass planets
- Find Earth-mass planets in Earth-like orbits where liquid water
- Search for signs of life, specifically biomarkers in the atmospheres
like O2 and O3 that we know are due to life
on our own planet.
For more information on the search for exoplanets and breaking news, try
- California & Carnegie Planet Search
- The Geneva
Extrasolar Planet Search Programmes
- The Extrasolar Planets Encyclopaedia
(a multi-lingual site in France).
- The Space Interferometry Mission
- Kepler Mission search for
planets, hopefully down to Earth-mass planets, using the transit method
from space. Currently scheduled for launch in February 2009.
- COROT Mission, a French
(CNES) mini-satellite launched in December 2006 to study stellar
oscillations and search for exoplanets using the transit method.
- Planet Quest at JPL.
A good source of information about NASA projects to look for Earth-like
- Extrasolar Visions is an
informative and imaginative page with some cool (if highly speculative)
- There are a number of consortia undertaking Gravitational
Microlensing searches, including an active group led by OSU:
- The MicroFUN Collaboration, home of a
gravitational microlensing search consortium coordinated by OSU
astronomers (including me). In summer 2005 we discovered our first planet by microlensing, with
the help of two amateur astronomers in New Zealand. Since then
our group has made crucial contributions to a number of microlensing
planet detections. This work is primarily funded by the
NASA Origins Program.
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Updated: 2007 November 26
Copyright © Richard W. Pogge,
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