Download Spectroscopy

Journal 2/9/17
What's the most alien kind of planet you can think of? Try
to come up with something realistic, but super-different
from anything in our solar system!
Objective
Tonight’s Homework
To learn how we find planets
around other stars
Read Nightfall pp 1-7
Detecting Extrasolar Planets
History and Detection:
The first exosolar planet was found in 1995 around
the star 51 Pegasi.
The planet is 150 times more massive than Earth but
orbits at 1/10 the distance mercury does from our
own Sun.
The planet is
designated “51
Pegasi b” but
has unofficially
been named
“Bellerophon”.
Detecting Extrasolar Planets
Since then, we’ve confirmed the discovery of
over 1,500 3,400 planets orbiting around other
stars.
But how do we detect them? We have 3
methods:
- Spectroscopy (red and blueshift like how we
detect binary stars.)
- Transit (when the planet goes in front of the
star, dimming it like a binary light curve.)
- Microlensing (when gravity from the system
can act like a lens, bending light around to
magnify possible planets in the system.)
Detecting Extrasolar Planets
The spectroscopic and transit methods are the
most successful, so we’ll discuss those.
When we look at a star, it’s possible to see some
very slight variations in the light curve. Since
planets go around stars only slowly, this
observation is usually over a period of years.
If we see slight
variation, it could
indicate a planet
making the star move
and wobble a bit.
years of time
Detecting Extrasolar Planets
We can use a similar method for spectroscopy.
If we see the star wiggle between red and blueshift over a period of years, we know planets
must be orbiting the star.
Determining the properties of a planet based on
these methods is far harder. After all, if we see
a wiggle, what does that tell us?
Scientists have managed to get approximate
size, distance, and temperature of a planet from
such wiggles!
Detecting Extrasolar Planets
We also have bias in the planets we find. Most
extrasolar planets are found around smaller stars (G,
K, and M classes). The planets we find are also
usually huge Jupiter-like planets.
This is mainly for two reasons:
1) Larger stars emit more radiation. This makes it
harder for planets to form because the light from the
star pushes away rocks and debris needed to make
planets.
2) Our instruments for detecting planets are sort of
crude. Because of this, we only really see the big
ones since they’re more visible. There are likely tons
of small planets we just can’t detect yet.
Detecting Extrasolar Planets
Even with this bias, we’re finding a lot of planets.
Astronomers estimate that on average, there are
about as many planets in the galaxy as stars. This
means there could be 400 billion planets in our
galaxy!
Homework Time
Your homework for this week is to read an award-winning
science fiction short story called “Nightfall”. It’s about a
civilization on a planet with 6 stars. It’s a pretty cool
story. Pages 1-7 are assigned for tonight, but you’re more
than welcome to read ahead.
Exit Question
True or false: the majority of planets we’ve seen, we’ve
observed directly with telescopes.
a) true
b) false