The search for extra-solar planets is one of the most popular and cutting edge research areas in astronomy.

To find answers to questions, such as:

Is our solar system unique?
Are Earth-like planets common?
Is there intelligent life elsewhere in the galaxy/universe?

would change how we view our place in the cosmos.
 

In 1995 astronomers discovered the first extra-solar planet.  By the end of 1996 a dozen extra-solar planets had been found and confirmed.
 

The discovery of planets around other stars

All extra-solar planets have been discovered using indirect means.  They have been detected by measuring their gravitational influence on the stars around which they orbit.  They have not been directly imaged.
However, in the case of HD209548 there is a transit of the planet in front of the star every 3.5 days causing a 1.5% drop in the flux of the star as seen from the Earth.

The kinds of planets that have been discovered are Jupiter like.  So far, astronomers cannot image such a planet because it is too small and faint compared with the star it orbits.  The glare of the star hides the planet.

As a massive planet orbits around a star each pulls gravitationally on the other and both the planet and the star actually move around the center of the mass of the system.  Consequently, the search for extra-solar planets has become a search for the back and forth motion (the "wobble") of the star as a planet orbits it.  This motion is known as the reflex motion of the star.

2 Ways to Detect Reflex Motion

1.  Using the Doppler Shift:  detect the changes in velocity of the Sun relative to Earth.

As the planet moves around the star, the star will move first toward us and then away from us.  Changes in wavelengths of light emitted from a star that result from its having a velocity either toward or away from us.

See here for a more detailed look

See here for an "Extra Solar Planets Encyclopaedia"!

We have found a great many stars (155 by March 2006) that have the radial velocity planets (see figure 10.13 in the text as well):

2.  Astrometric technique:  careful observations of the location of a star against the background sky can detect the wobbling reflex motion directly.

The more massive a planet is the more motion the star will display.  Smaller planets will produce reflex motion but the amplitude of the motion will be smaller.  Therefore, more massive planets will be detected first.
 

Many observations are needed to confirm the reflex motion.  Astronomers need to observe the possible planet several times in the period it takes to go around the star.  For e.g., Jupiter goes around the Sun once every 12 years.

*Therefore, planets that orbit closer to their central star (thus having the shortest orbital period) will be easier and quicker to detect.
 

What is the boundary between star and planet?

A few things about stars:

Binary stars are common--2/3 of nearby stars are binaries.

They form when a giant molecular cloud collapses and breaks into smaller clouds.  These smaller clouds keep collapsing and each forms into a separate star.  Sometimes these smaller clouds rotate around one another, resulting in 2 or more stars orbiting each other.
 
 

• Our Sun is a typical star and we know of other stars which are 20-50X the Sun's mass (20-50 M_s).

• Large stars have short lifespans (for e.g. 10 m.y.)--->they burn up quickly.

• Small stars--as small as only 8% of the mass of the Sun (0.08 M_s)--experience hydrogen fusion like the big stars because they possess a high enough central pressure and temperature.

• Objects smaller than 0.08 M_s cannot sustain nuclear reactions so 0.08 M_s are the smallest an object can be and still be considered a star.  These very small stars have been found in orbit around more massive stars.

A few things about Brown Dwarfs:

• Objects that collapse out of a molecular cloud with a mass between 0.01 and 0.08 M_s are considered to be Brown Dwarfs.

• Typically, brown dwarfs range between 10-50 Jupiter masses (M_J).

• They do not burn nuclear fuel but give off energy.

• Brown dwarfs are so named because they give out only meager amounts of light--hence brown dwarf.

• They can be thought of as "failed stars".

But until rather recently there had been no detection of these objects.  So far, astronomers have detected:

• a few brown dwarf companions---GL229B , the first detected brown dwarf companion (see image below).

• Using Adaptive Optics (see below) Dr. Close has found several new brown dwarf companions around low mass stars:


See here for a press release about Dr. Close's work on binary brown dwarfs (and an image of the closest brown dwarf ever seen around a star!

• 
• Here is an image of the closest binary brown dwarf ever found to the Sun (epsilon Indi Ba,Bb)

See here for a press release about Dr. Close's special SDI camera discovering another unique brown dwarf system very near the Sun!

• 4 planets orbiting pulsars (pulsars are the remains of a star gone supernova.  They rotate quickly and are entirely composed of neutrons--hence they are neutron stars) which are approximately the size of the Earth or Moon.
• 
• The rest of the planets discovered (~155 of them by March 2006) are Jupiter-sized or larger (see list above).

PLANETARY MIGRATION

One of the puzzling results of these discoveries lies with the Jupiter-sized planets because their orbit is close in to their central star, a location we expect to find more rocky planets like in our own solar system. We believe these "close jupiters" must have formed at 5 AU like our Jupiter but these other jupiters "migrated" towards their primary star.

Planetary Migration Simulation (Pawel Artymowicz) (movie)
 
 

Are extra-solar planets common?

Do most gas giants form in the outer solar system and stay there OR do most of their orbits evolve inward toward their central star?

Given the evidence, we don't know which is more typical.

Gas planets that orbit close to their central star probably would mean the existence of Earth-like planets in that solar system unlikely because of the gas giant either ejecting them out or pulling them in.

Earth-like rocky planets are probably common throughout the galaxy.
 
 

Are we close to directly detecting planets?

Direct detection of Planets with Adaptive Optics

Prof. Close is working on just that problem....

adaptive optics allows very sharp images to be made by correcting the "blurring" made by the atmosphere -in real time- with a "rubber mirror" that removes the aberrations -usually 1000 times per second!

See here for an "Introduction to Adaptive Optics"!

here is movie to show how Adaptive optics works

here is a movie to the improvement of Adaptive Optics ON compared to OFF on a
triple star.

here is an image showing how much better a star looks with adaptive optics

 

with AO prof. Close has found good candidates for the first direct images of planets around other stars:

Dr. Close has developed a even higher contrast technique for detecting extra-solar planets. He has deployed 2 of these "SDI" (simultaneous differential Imagers). Go here for more "first light" results from these direct planet detection devices.

Below is a movie of how Dr. Close's SDI camera can detect cool low-mass objects like an extra-solar planet (extra-solar planets will blink while residual light from the primary star is constant). Note how the low-mass object SCR1845B (discovered in these SDI images) is clearly blinking below:

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