Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 2:30

Lecture 45:
Exoplanets: Planets Around Other Stars

Key Ideas:

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?

Scientific Questions

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 planets (SETI).

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.

There are two manifestations of this motion: Astrometric Wobble and Doppler Wobble.

Astrometric Wobble

Parent star wobbles back & forth on the sky as seen relative to distant background stars.

Problem:

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 <0.001 arcseconds!

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.

Doppler Wobble or Radial Velocity (RV) Method

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.

RV 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.

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

So far, the RV method has been the most successful to date

About 85% of all exoplanets known thus far have been found using 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.

Planetary Transits

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 M_Sun and Radius of 1.18 R_Sun.
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.

Gravitational Microlensing

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 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 5M_Earth range.
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 parent stars
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 orbits.

What is going on? This is a subject of much current research.

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.

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Lecture 45: Exoplanets: Planets Around Other Stars