Astronomy 1101 --- Planets to Cosmos
|
Exoplanets
Key Ideas:
Search for planets around other stars.
Current Search Techniques:
- Direct imaging
- Doppler Wobble (Radial Velocity) Method via the Doppler Effect in orbits.
- 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,
is an old one.
- 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?
Searches for Extrasolar Planets
There are two basic search strategies:
Direct Detection
- 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.
Gravitational Detection
- Orbital motions ("wobbling") of the star because of the planet's gravity.
- Gravitational microlensing of a background star by the planet.
Wobbling Stars
Recall Newton's form of
Kepler's First Law of Planetary Motion:
- 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
greater mass.
Viewed from afar, the star will appear to wobble about
the center of mass of the star-planet system.
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 of Planetary Motion.
Radial Velocity 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
orbital speed.
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.
- The current state-of-the-art is 3 meters/sec
- New techniques can achieve <1 meter/sec!
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 observed wavelength of a wave will change when the source of the
waves and the observer are moving either towards or away from each other.
Examples:
- Sound Waves (Siren or Train Horn)
- Light Waves
The amount of the shift and its sign depends on
- relative speed of the source & observer
- direction of motion (together or apart)
Doppler Effect in Sound
Two cats are sitting between a windup mouse toy that emits an electronic
squeak. The mouse is moving towards the left towards the first cat and
away from the second:
(Graphic by R. Pogge)
The mouse emitted a squeak when it was at the location of each of the
green dots. The sound wave ("squeak!") moves outward spherically from
each point of emission. Because the mouse is moving, the sound waves
have different emission centers. Those waves ahead of its motion
(towards the left) are are scrunched together, while those behind are
spread out.
Results:
- The cat on the left hears a higher-pitched squeak because the waves
have a shorter wavelength (scrunched together by the mouse's motion).
- The cat on the right hears a lower-pitched squeak because the waves
have a longer wavelength (spread out by the mouse's motion).
Doppler Effect in Light
The Doppler Effect in light works the same way as it does for sound:
- Light source moving away from the observer, the
observed wavelength will get longer, and hence
REDSHIFTED
- Light source moving towards the observer, the observed
wavelength will get shorter, and hence BLUESHIFTED
A Way to Measure Speeds
Observe the wavelength
lobs of a
light source with a known emitted wavelength
lem.
The difference between the observed and emitted wavelengths is
directly proportional to the speed of the source towards or away from
you (v), given by the Doppler Formula:
(Graphic by R. Pogge)
Here c is the speed of light.
- The size of the shift gives the speed of the source
- The color of the shift (Red or Blue) gives the direction of motion
(away or towards you).
The Doppler Effect in Practice
The Doppler Effect in light is used by astronomers to measure the speeds
of objects moving towards or away from the Earth.
But, we also use the Doppler Effect in light in everyday settings. Some
examples:
- Traffic Radar Guns:
- Radar gun bounces a pulse of microwaves (or infrared laser light)
of a known wavelength off a car or truck, measure the wavelength
reflected back. The Doppler shift gives the vehicle's speed.
Most traffic radar guns are of the portable microwave doppler radar type in this example. A problem of microwaves is that their excess emission can be detected by small receivers mounted in cars and used to warn drivers that a radar gun is in use on the road. Some types of police "radar" guns don't use microwaves but instead use laser ranging techniques called LIDAR. This works by time-of-flight calculation rather than the Doppler effect.
51 Pegasi
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
days.
- 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
The RV method is very sensitive to massive planets around relatively
nearby stars
- 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:
- 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.
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 Case of HD209458
The first confirmed transiting planet. HD209458 is a star with a
Jupiter-sized planet found originally via the RV 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.
Advantages of Transits
Transits offer the only way we currently have to make a direct measurement
of the radii 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 only way we have to probe the atmospheres of exoplanets
- 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
composition.
- 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.
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.
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"
star.
- 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
(MicroFUN) collaboration.
Advantages of Microlensing
Microlensing is superbly sensitive to planetary systems like our own
Solar System:
- It is not biased towards finding close-in Jupiters like
the RV or Transit methods
It offers one of the few ways to find planets around more distant stars
- Aren't just limited to nearby strs
- Could give a more fair census of planetary systems
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.
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 2014, all of these techniques have found more than
thousands of planets or planet candidates. The first planets discovered were via the RV method. These found so-called "hot Jupiters" --- Jupiter-mass planets on very short orbits around their host star. Most of these systems have only one detectable planets.
Searches via transits with the Kepler Satellite have revealed thousands of potential planets. These range from Hot Jupiters (which are rare) to Neptune-mass objects, and even down to near-Earth masses on near-year orbits.
Strange New Worlds
Although microlensing has revealed one system with a Jupiter-mass and a Saturn-mass planet at the right semi-major axis to be an analog for our solar system, most of the systems discovered so far are not like ours. Many of the multi-planet systems discovered by Kepler are very "compact" in the sense that the bodies have small semi-major axes --- but, this could be just a "selection effect" since we have no way yet to find systems that are like ours via transits.
The Future
There are continuing searches for other planetary systems.
Basic Goals:
- Find systems more like our own Solar System
- Assess how common planetary systems are
Future Goals:
- Space missions being planned to search for Earth-mass planets
- Find Earth-mass planets in Earth-like orbits where liquid water
is possible.
- 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.
This is considered one of the most important astronomical research
programs of the 21st Century.
Supplement
For more information on the search for exoplanets 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
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
planets.
- Extrasolar Visions is an
informative and imaginative page with some cool (if highly speculative)
artwork.
- 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.
Updated: 2014, Todd A. Thompson
Copyright © Richard W. Pogge,
All Rights Reserved.