In this lecture we'll discuss why it's so
difficult to learn about extrasolar planets,
and how a star's motion or changes in its
brightness can reveal the presence of planets.
Our Milky Way galaxy has over 200 billion
stars.
Even if only a small fraction of those stars
have planets, our galaxy should still be bursting
at the seams with planets.
Current theories of star formation predict
that planetary systems are a natural consequence
of the process.
Therefore planets shouldn't be rare at all.
But it wasn't until the 1990s that we began
finding these planets.
The the simple reason being that detecting
extrasolar planets is a technical challenge.
Why the difficulty?
Well first, the stars are very far away.
Even if we are looking around the nearest
stars, it's like being in San Francisco and
trying to see a pinhead 15 meters from a grapefruit
in Washington, DC.
It's not so easy!
The second challenge is how bright the stars
are compared to their planets.
A Sun-like star is a billion times brighter
than the light reflected from any of its planets.
So imaging a planet next to a distant star
is also quite difficult.
But human ingenuity has prevailed, and we've
come up with clever ways to look for extrasolar
planets despite the challenges.
We look for planets in two general ways.
Directly and indirectly.
Direct detection means obtaining actual images
or spectra of the planets, and indirect detection
means inferring the existence of the planets
without actually seeing them.
We'll talk about indirect detection first.
Under the indirect umbrella, there are two
major approaches to finding and studying extrasolar
planets.
First we can observe the motion of a star
to detect the tiny gravitational effects of
the orbiting planets, and second we can observe
changes in a star's brightness that occur
when one of its planets passes in front of
the star.
The first indirect approach, the gravitational
one, is possible because stars and their planets
orbit around a common center of mass.
We usually think of one object orbiting another
object, like the Earth orbiting the Sun, but
objects attracted by gravity actually both
orbit around their common center of mass.
For example, in a binary star system with
which both stars have the same mass, we would
see both stars tracing ellipses around a point
halfway between them.
When one object is more massive than the other,
the center of mass lies closer to the more
massive object.
The idea that objects orbit their common center
of mass holds even for the Sun and planets.
It's just that Sun is so much more massive
than the planets so that the center of mass
between the Sun and any planet actually lies
inside the Sun!
Consider a star and single planet.
The star is still orbiting around the planet,
even if the center of mass is inside of it,
and the planet is still orbiting around the
star.
Also, the orbital period of the star is the
same as that of the planet.
The Sun's system is more complicated than
just one planet.
The Sun is orbiting around the combination
of all the planets and everything else in
the solar system.
If we were an alien species watching the solar
system from a distance, we'd see the Sun wobble.
And if we were really careful with our measurements,
and if we knew a little physics, we'd be able
to figure out there are multiple planets orbiting
around the Sun.
To find planets around other stars, we need
to therefore look for the motion of the star
caused by the gravitational tug of its planets.
We can look for this motion by making precise
measurements of the star's position in the
sky.
This is called astrometry.
Or we can measure the changes in a star's
velocity toward or away from us using the
Doppler effect.
Let's consider astrometry.
This is not an easy measurement to make.
For example, a Jupiter-size planet orbiting
5 astronomical units from a Sun-like star
would cause its star to move only about 0.003
of an arcsecond.
This approximately the width of a hair seen
from 5 kilometers.
Very tiny!
But it's not undoable.
The European Space Agency's GAIA mission has
already launched and is in the testing and
calibration phases.
It will measure positions of a billion stars
in our galaxy, some to an accuracy better
on the order of micro-arcseconds!
The Doppler technique also looks for the gravitational
tugs of planets around stars.
You may recall, the Doppler effect allows
us to measure the velocity of a star toward
or away from us.
When the star is moving toward us, the light
is blueshifted.
When the star is moving away from us, the
light is redshifted.
This technique can measure motions as small
as 1 meter per second, basically walking speed.
The Doppler method was used for the first
discovery of an extrasolar planet around a
Sun-like star in 1995.
The Doppler shift of the star 51 Pegasi indirectly
revealed a planet with 4-day orbital period.
This short period means that the planet orbits
very close to the star, about one-eighth of
mercury's orbital distance from the Sun.
This figure is an artist's impression of 51
Pegasi and its planet.
At distances so close, the planet must be
very hot!
In fact, the planet orbiting 51 Pegasi is
known as a "hot Jupiter", a planet with a
Jupiter-like mass but a much higher surface
temperature.
This is what the data look like for the Doppler
method.
This is a plot of the star's radial velocity
over time.
When the velocity is positive, the star is
moving away from us.
When the velocity is negative, the star is
moving toward us and the light is blueshifted.
At the beginning of the plot, the velocity
is positive and the star is moving away from
us.
As soon as it crosses the zero, the velocity
is negative and the star is moving toward
us.
Again, when it crosses the zero, the velocity
is positive and the star is moving away from
us.
This pattern will repeat as the planet orbits
around the star and the star wobbles.
Remember, this plot is the radial velocity
of the star and not of the planet.
The second general method is of detecting
planets indirectly is called the transit method.
If a star system is lined up so that we are
seeing it edge-on, then one or more of its
planets will pass directly between us and
the star once each orbit.
The result is a transit.
The eclipse reduces the star's apparent brightness
and can tell us something about the planet's
radius.
Because most stars exhibit some intrinsic
variations in brightness, we can be confident
that we have detected a planet only if we
observe the dips in brightness to repeat regularly.
The period of the repeated dimmings tells
us the orbital period of the transiting planet.
The goal of the Kepler mission is to discover
Earth-like planets in Earth-like orbits- worlds
that could potentially have life!
This is a technically challenging goal, but
Kepler has been enormously successful, finding
nearly 3000 planet candidates, some of which
are in that habitable zone.
Finally, this is a table from the text summarizing
the advantages and limitations of the indirect
methods I just talked about.
I don't want to spend a lot time here, you
can have a look-see for yourself at the different
techniques at the progress we've made so far
with planet detection.
That's it for now.
I'll talk to you more about extrasolar planets
soon.
