Astronomy 161: An Introduction to Solar System Astronomy Prof. Richard Pogge, MTWThF 9:30

Lecture 46:
ExoPlanets: Planets Around Other Stars

Key Ideas:

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?

• 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

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

Direct Detection:

• Take images of planets orbiting other stars
• Watch planets transiting their parent star, causing a characteristic drop in brightness.

Gravitational Detection:

• Effects of a planet's gravity on their parent star.
• Gravitational microlensing by the planet.

Wobbling Stars

• 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 wobble: Astrometric Wobble and Doppler Wobble.

Astrometric Wobble

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 Method

• 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

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.

51 Pegasi

• 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 (Geoff Marcy & Paul Butler) and Texas (Bill Cochran & Artie Hatzes).

Planetary Transits

• 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 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.

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.

Gravitational Microlensing

• 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, 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 amateur astronomers.]
• 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 other stars:

• 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 Saturn.
• 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 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?

Migration and Other Ideas

Migration:

• Jovian planets form far from the star
• Spiral inwards due to drag forces from the primordial stellar nebula material.
• Migration stops at the inner edge of the disk.
• Could be quite dramatic for very heavy stellar nebulae

Formation:

• 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 inwards?

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.

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-sized planets
• Find Earth-sized planets in Earth-like orbits where liquid water is possible.
• 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.

Afterword

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 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 Project Homepage 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.