Wednesday April 28th 1999
Announcements:

Lecture notes:

Other Planets

Last time we talked about the formation of the planets in a protoplanetary disk around the Sun.

These disks seem to be common for young stars suggesting that planets should be fairly common about stars.

In fact this appears to be the case based on recent discoveries since 1995.  We now know of at least 20 planets orbiting Sun-like stars and more are being found all the time.  We'll briefly discuss these today but only in basic details.

The first planets outside the solar system were found by PSU's Wolszczan around PSR 1957+12.  Has 3 planets 4.3 3.6 0.019 Earth masses at 0.19-.47 AU.

So for planets around Sun-like stars how do we discover them?

First of all we can't at present make direct images to see these planets.  Planets don't shine with their own light (at least in the optical band) and only have reflected light.  Expected to be greater than 1 million times fainter than their host star.

The main way that planets around Sun-like stars have been found is Doppler wobble.  Imagine we could make precise velocity
measurements of our Sun from far away.  We would see our Sun's velocity vary regularly by 12.5 m/s every 11.9 years.  Why?
Jupiter's gravity causes Sun to move back and forth - from this could deduce existence of Jupiter.  Binary star analogy.

Well scientists can now measure the velocities of other stars with about 3 m/s resolution.  They have found Jupiter-like planets
orbiting many Sun-like stars.

Some of these are quite bizarre.  The first found was 51 Pegasi b and has a mass of about .44 times Jupiter's mass but the orbit is only 0.005 AU from the star!  4.2 day period.  Remember Mercury is 0.39 AU from its star.  Quite surprising in light of
what we learned from the last class.  We learned that Jupiter-mass planets should only form far from stars since they
need icy material to build core quickly.

What is the answer to this?  Inward migration after formation? Many Jupiters?  Our theories are wrong?

There are several "51 Peg" type planets now.  We're lucky our Jupiter was not like this since it would eject Earth on its inward migration.  Also there are a few more with Jupiter-mass planets at more reasonable distances from their stars.

One system Upsilon Andromedae has recently been found with 3 planets.  .72 2.1 and 4.5 times Jupiter's mass.  Some have
eccentric orbits - multiple planet interactions and ejections.

Now Earth-mass planets do not have enough mass to make a detectable Doppler wobble.

To detect them we would need a way to directly image them.  As I said before difficult due to glare of star that overpowers
reflected planet light by over one million times.  Can't do it at present time but people at NASA think it may be possible with
infrared interferometry from space.  The Terrestrial Planet Finder.  Goal - conduct a complete census of Earth-like planets
out to about 15 pc.  Characterize atmospheres using spectra to look for water carbon dioxide ozone methane - search for signs of life.

Want it to go in about 15 years.  www.tpf.nasa.gov