What we know so far (October 2004)

• There are 136 confirmed extrasolar planets (ESPs, planets outside our Solar System), and probably planets orbiting about 7% of stars,
• The farthest ESP detected is about 17,000 light years away,
• The oldest ESP detected is about 12.7 billion years of age,
• The smallest ESP detected is about 14 times the mass of Earth, or approximately 8.4 x 10²⁴ kg.

    Most have a mass a few times less than that of Jupiter, orbit in an elongate ellipsoid closer to their host stars than Earth, and their orbits generally take less time at higher velocities. 

    The majority of host stars appear enriched with metals compared with our Sun, which is even slightly metal- enriched when compared to other stars.  Two popular hypotheses for this observation are that either planets were absorbed by these stars late in system formation, thus enriching (only) the stars' atmospheres, or that planets are more likely to form around more metallic stars. In addition, metallicity seems to increase with increasing mass. Another piece of host star interest notes that only 1% of solar- type stars (stars like our Sun) have short period planets (T < 20 days), whereas 10% of more metal- rich stars do.

How we know?

ESP detection methods include:
~Doppler searching/ radial velocity method- Changes in a star's radial velocity, that coming toward or away from the observer, imply the presence of an orbiting companion object having a gravitational effect on the star's travel. 
    This process also reveals a planet's orbital period and eccentricity (deviation from circular) of its orbit from the periodicity and rhythm of the velocity shifts.  The average orbital distance, or semimajor axis, can be figured from Kepler's third law [ T²= 4pi²/GM)(a³), G = grav. const, M = mass star, a = semimajor axis]. The planet's minimum mass can be calculated too, since the effective mass we observe is msini, where m =  true mass of planet, and i = angle of inclination of the planet's orbital plane to our view direction. This is necessary because we only can detect changes in the component of the planet's velocity along our line of sight (less than or equal to the true velocity). 

~Gravitational microlensing- The gravitational field of a planet and star system magnifies the light of a distant background star. This method allows low mass planets to be detected, but such observations cannot be confirmed because the correct geometry (lining up of planet/star system and specific background star) happens but once.

~Transit method/ astrometric transits- This method detects a planet's shadow as it passes before its star. It can be used for small planets and at very large distances, but again, only if everything lines up perfectly.

~Dust cloud examination- In the dust clouds of extrasolar systems, planets can leave traces of their gravitational influence as they travel though the clouds.  These clouds absorb the light of their stars and emit instead infrared radiation, which we can detect from space stations and such (but not from Earth's surface, because our own atmosphere absorbs infrared).

~Pulsar timing- Anomalies in the periodicity of pulsar pulses is another way to tell there is a planet nearby. This method is especially good for small planets.

How well do we know about ESPs?

    Because all of these methods are indirect, we cannot be absolutely sure of the assertions that stem from them.  And because these methods are indirect, there are occasionally different interpretations of the same data, and some might think certain data prove the existence of a planet, while others disagree.  Having said this, the math and physics and optics behind the search for ESPs is very precise, and assertions made are generally accepted by the scientific community.

Some big names in the field

~Aleksander Wolszczan- This Polish radioastronomer/ astrophysicist is one of the foremost pulsar researchers and helped discover the first extraterrestrial planetary system around a pulsar.
~Steinn Sigurdsson- An Icelandic ESP scientist specializing in gravitational physics, compact object dynamics and related issues.
~Michel Mayor and Didier Queloz- A Swiss duo that discovered the first ESP around a solar- type star, and went on to discover many more ESPs.
~Debra Fischer, R. Paul Butler and Geoffrey Marcy- trio that has discovered many ESPs,  developed the method of highest precision Doppler measurement and made the first Zeeman magnetic field measurements.

Other conclusions and applications to our Solar System

    The prevalence of highly elongate orbits suggests against condensation- out- of- disk as a theory of planetary formation, but possible explanations for this observation include the original presence of two planets gravitationally interacting to cause elongate orbits, only to have one leave the system or be absorbed into the star or, second, that the planets originated as icy cores in the interstellar cloud, which requires no spherical orbit.                
    However, there is a gap in the mass distribution between the smallest brown dwarf [object too big to be a planet, but with not enough mass to be as nuclear as a star] and the largest planet, and this gap is consistent with the theory that stars form in interstellar clouds and planets form from circumstellar disks.

    Also, it seems that in multiplanet systems, the planets may not orbit in the same plane, as they do in ours, one more disparity between our Solar System and ESP systems found thus far.

    Clear protodisks have been observed in telescoped interstellar clouds, supporting the mainly accepted theory of ESP system formation, and in addition to the discovery of planets like Jupiter with a Jupiter- like orbit, and stars like our Sun, as well as the number of unexplored stars in the universe, it seems reasonable to assume that although we have not as yet found a solar system like our own, they are surely out there.

A Few Fun links

~Extrasolar Planet Encyclopaedia. This is a great comprehensive site, and the home page always has current ESP news.
~Drake Equation, which estimates the probability of communication with extraterrestrial life.  Good food for daydreams...
~Table of ESPs and properties, current to March, 2004.
~NASA interactive gallery on ESPs.

Picture/ Figure gallery


The following histograms are of planets discovered
before June 2001. Done by Debra Fischer.



               









"Springtime" on a large, Earth sized moon
of planet b, as imagined by Christopher
Kulmann. Artwork from Exoplaneten.de.


Depiction of a large, close ESP.


High resolution imaging of section of Orion Nebula.






Another Orion Nebula image.


Yet another cool Orion Nebula image.


Carina Nebula image, where "protoplanetary cocoons of gases" have been seen.

Okay, now that your appetite has been whetted, continue your image and information
search on your own. I promise you will find many more items of interest!

Site by Laura Hitchings, for Catherine Johnson and her Geodynamics and Terrestrial
Planets class at University of California, San Diego in association with Scripps Institute of Oceanography.
Last updated October 7th 2004.