An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30
ExoPlanets: Planets Around Other Stars
Search for planets around other stars.
Current Search Techniques:
- Astrometric Wobble
- Doppler Wobble - most successful so far
- Planetary Transits
- Gravitational Microlensing
Extrasolar planetary systems:
- Jupiter-sized planets close to their parent stars.
- These challenge solar system formation models.
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
- 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 speculation.
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
Searches for Extrasolar Planets
There are two basic search strategies:
- Take images of planets orbiting other stars
- Watch planets transiting their parent star,
causing a characteristic drop in brightness.
- Effects of a planet's gravity on their parent star.
- Gravitational microlensing by the planet.
Recall Newton's form of Kepler's 1st Law:
- 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
Viewed from afar, the star will appear to "wobble" about
the center of mass of the star-planet system.
There are two manifestations of this wobble: Astrometric Wobble and
Parent star wobbles back & forth on the sky as seen relative to more
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.
- 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.
Measuring the orbital motions provides an estimate of the unseen
planet's mass via Newton's form of Kepler's third law.
Doppler Wobble Measurements
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
- 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
The challenge is to measure the Doppler shifts of the lines with
extremely high precision.
To convey some idea of the scale of the problem, a person can walk at
a speed of about a meter per second, while a car moving 65 MPH is moving
at 29 meters/second.
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 (Geoff Marcy & Paul Butler) and Texas (Bill
Cochran & Artie Hatzes).
If the orbital plane of an extrasolar planet is aligned with
the line of sight:
At present, there are about 5 cases of candidate transiting planets.
- 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
A star with a Jupiter-sized planet
found via the Doppler Wobble method:
- 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
Using an orbit prediction from the Doppler work, astronomers observed
HD209458 and were able to see the planet transit its parent star.
A number of searches are under way for more transiting planets, and at
least three more have been found so far. OSU is engaged in a couple of
different searches for transiting planets, but no discoveries to date.
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, two planets have been found by gravitational microlensing
by the OSU team
- 2005: Jupiter-mass planet found by
the OGLE, MicroFUN, MOA, and PLANET/RoboNet collaborations. [OSU leads
the MicroFUN group, which included key observations by two New Zealand
- 2006: Neptune-Mass "Super Earth"
planet found by MicroFUN, OGLE, and the PLANET/RoboNet collaboration.
Roster of New Planetary Systems
As of November 2006, various techniques have found more than
200 planets with masses of <18 Jupiter Masses around around
- Most planets are <10 Jupiter Masses
- Most are single giant planet systems
- A large number of multiple-planet systems have been found so far
- Most are Jupiter-sized or larger (up to 13 times Jupiter's mass),
with some recent detections getting into the Neptune range, one
one estimated at 6-8 Earth Masses (a "super-Earth").
- All orbit within ~5 AU of their parent star.
Some caveats are in order:
- Doppler Wobbles are only detectable for large planets,
since the technique insensitive to speeds slower than 3
meters/sec. This is the wobble induced in the Sun by
- The masses estimates assume the orbit is oriented exactly
edge on, giving the smallest mass that could make
the observed wobble.
- The Doppler technique is currently only sensitive to Jupiters
closer than ~4 AU from their stars. Jupiter, for
comparison, is 5.2 AU from the Sun
- The transit technique is strongly biased towards finding the
very close-in Jupiter-like planets (these give the
highest probability of a transit).
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?
Migration and Other Ideas
There are a couple of possible explanations:
- Jovian planets form far from the star
- Spiral inwards due to drag forces from the primordial stellar
- Migration stops at the inner edge of the disk.
- Could be quite dramatic for very heavy stellar nebulae
- Close-in Jupiters formed differently than in our Solar System
- Might be different in detail than Jupiter or Saturn.
The migration idea is currently the subject of intense research. For
example, why didn't the 4 Jovian planets in our Solar System migrate
Part of the answer is that they have migrated at least a little, since
we see asteroids swept into resonances (Kirkwood Gaps), and the Plutinos
swept into 3:2 resonance with Neptune. For whatever reason, migration
was nowhere near as extreme as implied in these other systems.
The theoretical and observational picture is still very confused, but we
are making exciting progress in trying to understand what is going on.
There are continuing searches for other planetary systems.
- Find systems more like our own Solar System
- Assess how common planetary systems are
This will be one of the most important projects of the 21st Century.
- Space missions being planned to search for Earth-sized planets
- Find Earth-sized planets in Earth-like orbits where liquid water
- Search for signs of life, specifically markers in the atmospheres
like O2 and O3 that we know are due to life
on our own planet.
Recalling the caveats mentioned above, the Doppler Wobble experiments
are most sensitive to systems with large planets close to their stars,
but cannot (yet?) tell us about planetary systems like our own. To do
that, the experiment would need to achieve at least 1 meter/second
precision, and be carried out over a much longer time. Recall that
Jupiter is 5.2 AU from the Sun with an 11.2 year orbital period. To get
an orbit, you need to observe the system for at least 1, and preferably
2 orbits, which would require at least 11 years of data, and better, 22
years of data. All of the Doppler Wobble experiments have been going on
since 1988, and not really at full precision until the last decade. We
still don't know if the systems we have detected are representative, and
therefore do not know if our solar system is a rarity or commonplace.
Understanding the "selection effects" in all of these studies in an
on-going (and contentious!) undertaking.
For more information, and breaking news, try these websites:
- 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 Earth-size
planets using the transit method from space. Currently scheduled for
launch in October 2008.
- COROT Mission
mini-satellite to study stellar oscillations and search for exoplanets
using the transit method, being prepared for a December 21, 2006 launch
from Baikonour Space Center.
- 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
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.
Finally, a nice but fairly technical overview of changing theories of
planetary systems is given on this page by
Pawel Artymosicz of the Stockholm Observatory. The historical
overview is the most accessible to Astronomy 161 students.
Readings in Universe: None. You can
look at Section 7-9 if you like, but it is relatively out of date.
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Updated: 2006 November 28
Copyright © Richard W. Pogge,
All Rights Reserved.