Download Theme 7.2 -- The Complete Solar System

Theme 7.2
--
The Complete Solar System
Expected Outcomes of the Formation Process
In the previous unit we considered the nebular hypothesis for the
formation of the planets. At the end of the episode of planet
formation, we might expect to see certain things in the solar system.
First, leftover gas that had not participated in the formation
process.
Second, evidence of large collisions that took place late in the
process as the biggest lumps were merging finally to produce the
planets.
Finally, leftover rubble that might date back to the time of the
formation stage: unaccreted lumps that never did make it into the
planets.
In addition, we'd expect to find evidence that it all happened
about the same time. This will require us to develop tools for
measuring the age of the different parts of the solar system.
Furthermore, if the nebular hypothesis is correct, we'd expect to see
evidence of planetary formation around other stars. This could take
two different forms: first, we might see gaseous discs surrounding
young stars in which we would interpret the formation process to be
continuing: planets being formed as we watch. Secondly, we might
expect to find large numbers of established planetary systems
comparable to our own solar system.
On the other hand, we have to be aware of the possibility that things
may have slowly changed since the formation process itself. The solar
system may not look entirely now as it did at the time of formation.
Some of the evidence may have been eradicated, or there may have been
slow, gradual changes in the arrangement. These possibilities must be
considered.
Leftover Gas: the Earth’s Atmosphere
The first and most obvious of these is the question of leftover gas. We
do not see the planets moving through a gaseous medium -- even the
innermost planets, where we argued that not all the gas would have
condensed. What happened to the leftover gas? Why is it no longer
present? The solar system is largely a vacuum. Where did that gas go?
Pertinent evidence comes from our study of what are known as T Tauri
stars, named after a prototypical star in the constellation of Taurus.
These are young stars of about the same mass as the sun, and they are
observed to go through an early stage where they have enormously strong
stellar winds. The present sun has a modest ‘solar wind’ which is
responsible for things like the Northern Lights, but the stellar winds,
the T Tauri stars are enormously greater, and would have swept out all
of the leftover gas, and would even have scoured off much of the
earth's primitive atmosphere.
A detailed study of the composition of the earth's atmosphere reveals
that it is secondar,y created somewhat later after the formation of the
planet itself by outgassing from volcanos and from the gradual
accumulation of the vaporization of incoming material, including grains
and pebbles, comets that are rich in icy material and so on. Indeed it
is even possible that much of the water now on the earth was carried in
by icy bodies such as comets.
Not only is the earth's atmosphere secondary, but it has also undergone
considerable evolution over the billions of years since the planet
itself formed. One of the main reasons for this, of course, is the
emergence of life, several billion years ago, which led to the
absorption of carbon dioxide from the original atmosphere and the
production of free oxygen which is now present in 20% abundance and
support our existence.
The Bombardment History
The nebular hypothesis suggests a second observable consequence of the
planet formation process. In the early stages, small pieces would
emerge together relatively quietly, but towards the end as the final
planets are emerging, there might have been substantial major
collisions between rather large lumps, and we could look for evidence
of such events. On the earth, we undergo active geology and weather,
and that quickly erodes away any evidence of impacts, but we can look
at more primitive surfaces as on the airless moons and the planet
Mercury to see what we can determine from the evidence in the form of
the size, number and distribution of impact craters that write the
bombardment history of the solar system.
The best evidence for this comes from our study of the moon, because
we've managed to bring back lunar rocks which can be age-dated very
carefully to pin down the timescale of the various episodes. The
diagram here shows that in the early solar system the bombardment rate
was very high, gradually dwindling down as the pieces accumulated, but
then about 3.8 or 3.9 billion years ago, there was an episode known as
the “late heavy bombardment.” Since then it has levelled off, although
of course, there are still many pieces in orbit that could cause
catastrophic impacts yet.
Major Collision Events
In addition to these generalized remarks about the bombardment history
in the solar system, we now recognize certain circumstances that may be
attributable to some individual large impact events. For example, the
fact that Venus spins in a retrograde direction maybe attributable to a
late impact with an object of fairly considerable size, and likewise
the fact that Uranus is tipped on its side as it spins. Indeed it is
now generally believed that the formation of our own moon maybe
attributable to a major impact event between the earth and a Mars-sized
piece as we'll see on the next panel.
The earth is unique among the terrestrial planets in having a moon that
is rather large compared to its own size, and one of the ways of
understanding this is in the model shown here, in which there's an
early impact between the earth and a Mars-sized object coming in a
fairly high velocity. This leads to the tip of the earth, and the
formation of the moon in the way that you see it here. As noted, this
is a plausible but not yet compelling argument, with evidence that
depends on our detailed understanding of the chemical composition of
the moon and the earth itself. We won't go into the details here,
however.
Leftover Material
A last consideration from our nebular model is to recognize that not
everything would have gone into the planets. There must be leftover
pieces, and indeed that's what we see: the solar system is full of
meteoroids, comets, asteroids, small objects out beyond the orbit of
Pluto, and so on. This will be part of our discussion in a later
theme.
Ages
Finally,
model is
all have
where we
system.
we recognize that one of the clear implications of the nebular
that the planets, comets, meteors, asteroids, and so on should
roughly the same age. We'll explore that in the next section
talk about age-dating techniques and the age of the solar
Extra-Solar Planets
If the nebular model is right, then we expect other stars to have
planetary systems in abundance. As noted, these could be systems that
are forming now, or systems that are long-established. Let's first ask
whether we see evidence of protoplanetary discs around nearby stars.
Disks of Gas and Dust
Remember, that this material will be cool, not glowing in the visible
at all, so we will have to use infrared or even radio radiation to hunt
for such discs of gas and dust around nearby stars. The first such
success came more than 30 years ago, thanks to observations made by the
Infrared Astronomical Satellite IRAS. Here we see the star Beta
Pictoris. The light from the central star has been blocked out here,
but we can see the extended disc of gas and dust glowing in the
infrared. Not much detail if visible, but the detection seems clear
cut.
We have much better technology now and can observe in considerably more
detail. Here we see the star HL Tauri, which has an estimated age of
about a million years. This is as seen by the new ALMA telescope in
Chile, which works at millimeter wavelengths. Notice the disc of cool
material, gas and dust; there are gaps where protoplanets are inferred
to be sweeping up the material as this planetary system forms.
Established Planetary Systems
The nebular hypothesis also tells us that there should be established
planetary systems around many nearby stars. Here, we consider various
detection techniques that could lead us to those discoveries.
We might hope to get direct images of planets as dots of light.
This is very challenging for planets shining by a reflected light next
to a bright parent star but it is possible for a few.
As the planet orbits thanks to Newton's Laws we know that the
star itself should wobble in response. The wobbling of the star can be
measured in the fore-and-aft direction as the star moves towards and
away from us, thanks to the Doppler shift in the spectral lines, but it
is not yet possible for us to measure the side-to-side motion of the
star in response to the planet's presence.
And finally, if a planet should pass briefly in front of a star
in a transit, we expect to see a dimming of the star, and of course if
the planet is in regular orbit we would expect that to be a repeated
periodic effect.
Direct Imaging
Direct imaging of planets is very challenging, as noted. It's
comparable to looking for a firefly hovering beside a searchlight. The
planets are shining by reflected light and only a little will be cast
in our direction compared to the bright star.
Here, though, we see a
system in which the light from the star itself has been masked off and
three planets are indeed visible. Such examples are so far very rare
though.
‘Velocity Wobbles’
In an earlier unit we talked about how we detect the presence of
planets through the velocity wobbles as measured by the Doppler shift
in the absorption lines. On the left, we see a reminder of why this
happens. The planet is making a large orbit around the common centre
of mass, the star wobbles a little and is sometimes approaching us,
sometimes receding. On the right, we see the observed change of
velocity periodically as the star moves back and forth from our
perspective.
Transits
Earlier, we explored the use of transits in the same respect. On the
top left we see what is happening, with the planet moving across the
face of the star and blocking off some of the light. We do not see
that detail but we notice the dimming as shown on the bottom left where
there's loss of a little over a percent of the light for a short period
of time. On the right-hand side we see how much better we do from space
where we don't have the problem of the variable earth's atmosphere
confusing the measurements. The bottom part of that diagram shows the
measurements that are made with the Kepler telescope in space that has
been studying many hundreds of thousands of stars for a long period of
time.
Here indeed is a reminder of the operation of the Kepler space
telescope, which was put into space to look fixedly in one direction to
study more than 100,000 stars continuously to try and detect transits.
On the right, we see several transits of particular stars showing the
duration and the dimming of the light, and the link at the bottom leads
you to a very nice brief animation in which we see how Kepler can
detect multi-planet systems, where the flickering of the star tells us
about the presence of 5 or 6 planets in a single system.
The discovery of so many planetary systems is very strong evidence that
the nebular hypothesis is correct, and that most stars will have
abundant planets surrounding them. In this panel, we see some
particularly interesting planets: those that are not very much bigger
than the earth and may be in regions around the parent star in which
they could provide habitable zones for life forms to exist, where it's
neither too hot nor too cold. This is a topic we'll return to when we
talk about the search for extraterrestrial life.
Selection Effects
So, we've discovered planetary systems around many stars but we should
be aware of very strong ‘selection effects’ -- that is to say certain
biases that are going to influence the kinds of planets we can detect
and constrain our ability to draw general conclusions. For example,
planets that are big in size are the easiest to find because they block
off more light during transit. Planets that are large in mass are
likewise easiest to find because they make the parent star wobble more
in velocity. Finally, planets that are closer to the star are easiest
to find because they're more likely to produce a transit, and they also
have stronger gravitational effects if they're close. This means that
big, massive planets close to the parent stars are the most easily
picked up, at least to begin with in these studies. Given all that,
it's not surprising that many of the planetary systems that we've
detected so far contain large, massive planets quite close to the
parent stars. It will take many years and improving technology to
allow the confirmed detection of a solar system very much like our own.
In the top panel we see a summary -- now a little dated -- of the early
detections of extra-solar planets. We see that indeed the big planets
are very much in the dominance here, but that is slowly changing with
time, as we discover more and more small planets, which requires high
sensitivity and time. In the bottom panel, we see a particular system
known as Upsilon Andromedae where there are three planets that are
comparable to or larger than Jupiter in size, and yet they're all
relatively close to this parent star. At the bottom of that panel we
see our own solar system for comparison.
Planetary Migration
It's not surprising that we are able to find the big planets, but the
real question is why are there so many Jupiter-like planets so close
the parent stars? Consider Upsilon Amdromedae, the system we looked at
on the last panel, for example. In the nebular model, we expect only
small rocky planets to form in that region. It's so hot there that
only a fraction of the initial material can condense, so the inner
planet should not be huge -- and yet here we find three planets
comparable to or larger then Jupiter very close to the parent star.
How can that be explained?
From the study of such systems, a new understanding has emerged. We
now realize that complex gravitational interactions between planets can
cause them to migrate, that is to say, to move around in a planetary
system over the passage of many millions of years. Small planets may
even be ejected from the system entirely. So, the system as we see it
now is not as it was at the time of formation: planets have drifted in
their positions.
Computer modelling now suggests that effects of this sort may have been
important in our own solar system as well. The big outer planets may
have migrated to some extent, and there would have been important
effects on the orbits of the asteroids, the many small objects beyond
Neptune, and possibly Uranus and Neptune themselves. By good fortune,
however, the earth's orbit has been relatively stable, and life on
earth has survived, of course, we would not be here to discuss it if
the situation were otherwise. Other planetary systems may not be so
fortunate.
So, this raises yet another consideration in our hope of detecting
extraterrestrial lifeforms elsewhere. They may, like us, need to be
lucky survivors.
An important goal in our study of extrasolar planets is to be able to
isolate the light of a single planet orbiting a nearby star and look
for spectroscopic signatures of atmospheric components that suggest
that life may be present -- for example, oxygen. The new James Webb
Space Telescope (the replacement for the Hubble Space Telescope) has
set this detection as one of its challenging aims. The link at the
bottom of the page will take you to a site that allows you to explore
the possibilities.