Download Other Planetary Systems

Theme 7 – The Complete Solar System;
Other Planetary Systems
ASTR 101
Prof. Dave Hanes
After the Formation of the Planets
We might expect to see in the Solar System:
1. leftover gas
2. evidence of large collisions that took
place as the biggest lumps merged
3. leftover ‘rubble’ dating back to the
formation stage (unaccreted lumps)
- plus evidence that it all happened at
about the same time
Around Other Stars…
…we might also expect to see evidence of such
processes, producing much the same results.
These could be in the form of either:
 gaseous disks (planets forming right now)
or
 established planetary systems (from long ago).
But We Must Aware of…
… the possibility that some things may have
significantly changed (either in the SS, or in
other systems) since the formative stages.
Some important evidence may have been
eradicated; or the ‘final’ arrangement may
have slowly evolved over billions of years
1. Leftover Gas?
We do NOT see this:
Instead, the planets travel through mostly
empty space, so any leftover gas is long gone.
But H and He could not have condensed in the
inner Solar System. Where did that gas go?
’T Tauri’ Stars
Young stars of about the same
mass as the Sun go through an
early stage where they have
enormously strong stellar winds
consisting of charged particles.
This can sweep out all the
leftover gas (and even scour off
much of the Earth’s primitive
atmosphere!).
Earth’s Atmosphere
It is secondary, created somewhat later by
 outgassing from volcanoes
 gradual accumulation from the vaporization of incoming
grains, pebbles, meteors, comets (rich in icy material), etc
(It is even possible that
much of the Earth’s water
was carried in by comets!)
It Has Also Evolved Subsequently
Note the decrease in Carbon Dioxide with the emergence
of plant life – and the growth in the abundance of Oxygen!
2. Evidence of Major Collisions
On Earth: our active geology and weather quickly erodes
away any evidence of impacts. Not very helpful!
Moon
Mercury
Vesta (asteroid)
But on older airless surfaces (the Moon, Mercury, other
small bodies): the ‘bombardment’ history is recorded in the
distribution, size and number of impact craters.
The Record on the Moon
Evidence of very heavy impact rates ~4.5 billion years ago,
then a brief period of ‘late heavy bombardment’ about 3.9
b.y. ago that has since levelled out.
Some Really Big Impacts
Major impacts may have
been responsible for
 the retrograde
(backwards) spin of
Venus
 the fact that Uranus is
‘tipped’ on its side as it
spins
…and possibly also the
formation of our Moon!
The Formation of the Moon
- some very strong evidence for this, but not yet compelling
3. Leftover ‘Rubble’ in the SS
The Solar System is full of meteoroids,
comets, asteroids, small objects out beyond
the orbit of Pluto, and so on.
That is, not every chunk wound up being
accreted by a growing planet!
We return to this topic in a later unit.
When Did This All Happen?
In the next section, we will discuss age-dating
techniques and the age of the Solar System.
Other Planetary Systems?
First, do we see evidence of proto-planetary disks
around nearby stars?
Remember than this distributed material will be
cool, not glowing in the visible. So we use infrared
(or even radio radiation) to hunt for such disks.
Yes! The Star Beta Pictoris
as seen in 1983 by IRAS
(the Infrared Astronomical Satellite)
(the light from the bright central star has been blocked off
here)
Much Better Technology Now
The star HL Tau (with an
estimated age of about
1 million years) as seen
in 2015 by the new
ALMA telescope in Chile
Notice the disk of cool
material (gas and dust)
with gaps where protoplanets are sweeping up
material.
Established Exo-Planetary Systems
A reminder of various detection techniques:



Get direct images of planets as dots of light; this is very
hard next to the bright parent star! Now possible for a
few.
As a planet orbits, we detect the ‘wobbling’of the star
(a) towards and away from us – measurable in
the spectral lines, using the Doppler shift
(b) from side to side – not yet possible
As a planet passes briefly in front of the star (a transit),
we see a repeated periodic dimming of the light
Direct Imaging? Hard!
The star is bright; a planet is faint
(Imagine a firefly beside a searchlight!)
Three planets are seen here.
(The light of the star is masked.)
Detecting Velocity Wobbles
[the first discovery of exoplanets]
A reminder of why
this happens:
Transits Can Be Studied
Better data from space!
The Kepler Space Telescope Detected Many!
http://www.youtube.com/watch?v=OKE8Jv0WhSM
Recent Interesting Findings
Beware Strong ‘Selection Effects’
Planets that are big in size are easiest to find: they block
off more light during transits
Planets that are large in mass are easiest to find: they
make the parent star ‘wobble’ more in velocity
Planets that are closer to the star are easiest to find: they
are more likely to produce a transit, and have stronger
gravitational (‘wobbling’) effects
This Explains Why…
…in many of the planetary systems detected so
far, we find big, massive planets quite close to the
parent stars (especially with the ‘wobble’
technique; using transits is better able to find
smaller planets.)
It will take many years, and improving technology,
to allow the confirmed detection of a Solar System
like our own.
Lots of Big Planets
Here, overall discoveries.
Notice the bias in favour
of big planets
The Upsilon Andromedae
system has three Jupiterlike planets very close to
the star!
One Puzzling Aspect
Finding big planets is not so surprising, but
why are so many ‘Jupiters’ so close to the
parent stars?
In the nebular model, only small rocky planets
should form in that region. (Only a fraction of the
initial material can condense where it is hot, so the
inner planets shouldn’t be huge.)
A New Understanding
Complex gravitational interactions between planets
can cause them to ‘migrate’ (move around in the
planetary system) over the passage of many
millions of years.
Smaller planets may even be ejected from the
system entirely.
Lucky Us!
Computer models suggest that the big outer
planets in our own Solar System may indeed have
migrated to some extent, with important effects on
the orbits of the asteroids, the many small objects
beyond Neptune, and Uranus and Neptune
themselves.
But by good fortune the Earth’s orbit has been
relatively stable, and life on Earth has survived.
Other planetary systems may not be so fortunate…
One Goal
Look for signs of
life, like a planetary
atmosphere rich in
oxygen (as indicated
by its spectrum)
To Find Out More…
The James Webb Space Telescope has set
this detection as one of its aims.
http://ngst.gsfc.nasa.gov/