- The origin and evolution of the solar system. We've already discussed
overall layout of the solar system, and a simple model for its formation
that explains the motions of planets around the sun.
- Review: formation of solar system from an interstellar gas cloud.
We've used this model to understand:
- how the planets get the transverse velocity needed to keep them
orbiting around Sun
- why all the planets orbit in the same direction (and in roughly
- why all the planets orbit in the same plane
- However, we've also learned that the planets are not all identical
to each other. In particular, there seem to be two or three main groups
of planets: the inner rocky planets, the outer gaseous planets, plus
Pluto (and Kuiper belt objects). Now we'd like to consider:
- How do we know that planets fall into these different groups?
- Can our model for the formation of the Solar System explain why
the different groups exist?
- How do we know that there are different groups of planets?
- We found that planets do not all have the same mass, but come in a
wide range of masses, by studying orbits of moons. We can start by asking the
question: how do we know that the more massive planets are not simply bigger
versions of less massive ones, and that they are made of something different?
We then want to understand why this might be.
- We can see that the outer compositions of the planets are different
from each other. What about the interior compositions?
- Consider a
do you think?
- These questions can be approached approximately by measuring the
densities of the different planets; density measures how
tightly packed together the material of an object is. The density
can provide a clue about what an object is made of. The density of
an object is defined as:
density = mass/volume
Densities are relatively easy to measure; we can measure masses using
the motions of satellites and our understanding of gravity, and we can
measure volumes by observing that planets are roughly spherical and by
measuring their diameters.
- When we measure mean planetary densities, we find that planets do
not all have the same density. The inner four planets have densities
of about 5gm/cm3 , while the outer planets have densities of about
1gm/cm3 . Pluto doesn't quite fit with the other; it has a density of
about 2gm/cm3 . To get a feel for what these numbers mean, note that
water has a density of 1gm/cm3 . The units, gm/cm3 , can be understood
by noting that the density in gm/cm3 just gives the mass of a small
cube of the material which is 1cm on each side.
- We see that planets are split into 3 classes: the inner planets
(terrestrial) which are more dense, the outer planets (Jovian) which
are less dense, and Pluto (and other Kuiper belt objects),
which is intermediate.
- Remember that these measurements just refer to mean densities
of planets. In fact, planets are not made of uniform density material;
they are more dense in their inner regions, and less dense in their
- Since the densities of the planets are different, we infer that the compositions of planets differ. The inner planets are made of rocky
material, which has higher density; these are often known as terrestrial
(earth-like) planets. The outer planets are made mostly
of low density gases; these are often known as Jovian (Jupiter-like) planets.
Pluto may be made of some combination of rocks and ice, and may represent
one of the largest of a third category of objects, called Kuiper belt objects,
that we discussed a bit in the first unit of the class.
- Why are the compositions of planets different from each other?
What determines the composition of planets?
- The reason for different composition of planets has to do with how
the solar system formed. In the early solar system, there was a disk
of material rotating around the Sun, from which the planets eventually
formed. However, to form planets required some kind of initial clump in
the protoplanetary disk. These clumps were probably formed from collisions
of solid particles. Different elements become solid at different temperatures:
the most common elements, hydrogen and helium, never became solid, but
hydrogen compounds (like water) can solidfy at colder temperatures, like
those in the outer solar system. In the inner solar system, it was too
hot for these compounds to solify; only rocks and metals can solidify at
these temperatures. Hence, only small planetessimals formed in the inner
- In the atmospheres of planets, atoms are held to planets by gravity.
However, if an atom can move fast enough, it can escape the gravitational
pull of the planet, in the same way that we can launch spacecraft which
can escape the gravitational pull of the Earth by shooting them off
- In fact, atoms do move around. The amount they move is related to
their temperature. In a hotter environment, atoms move faster. However,
the speed that atoms move also depend on the type of element; at the same
temperature, heavier atoms move slower than lighter atoms.
- These properties can be used to generally understand why the
compositions of the inner planets differ from those of the outer planets.
- The outer planets are sufficiently massive and sufficiently cool that
no elements can escape their gravitational attraction, so their composition
is similar now as to what it was when they originally formed. This is also true
of the Sun. This primordial composition is the same as that of most of the
Universe: about 90% hydrogen, 9% helium, and only 1% for everything
else combined. The normal matter in the Universe is mostly hydrogen.
- The inner planets are less massive and hotter than the outer
planets, so many elements can escape their gravity. On Earth, hydrogen
and helium escape so they are not an important constituent of our
atmosphere. Instead, our atmosphere is mostly nitrogen and oxygen.
- On the smallest objects, e.g., Mercury and the moons of most
planets, gravity isn't sufficiently strong to retain any atoms -
so these objects don't have any atmospheres at all.
- Although these principles are the most important in determining the
composition of planetary atmospheres, other effects can also be important,
such as the presence of volcanism, life, or manmade pollution.
- Although the inner planets are roughly similar to each other, and
the outer planets are similar to each other, there are some important
differences, especially, for example, if one wanted to consider a
visit to other planets. The details of the nature of each planet is
different. Also, planets are not totally uniform (they're not the
same in the center as on the outside), and they change as time passes.
So we can talk about the evolution of planets, we will discuss physical
processes that are important on each planet.
- First let us consider the
The planet we know most about is our own planet, Earth.
surface of Earth
is mostly composed of water, which has a density of 1gm/cm3 . Even
if one considers just the surface rocks, one only finds typical densities
2.7gm/cm3 . Since the global density of earth is
we immediately know that the inner parts of earth are different from the
surface. The central regions of Earth are significantly denser. We believe
that the central regions of Earth are composed primarily of iron-like
elements, while the surface rocks are composed of lighter elements.
- We can learn about the internal structure of Earth using earthquakes.
vibrations through the Earth,
and by measuring how long it takes these vibrations to pass
through various parts of the Earth, one can learn a lot about
the internal structure of the planet. We find that the Earth has
three main layers:
a core, a mantle, and a crust. The core is probably hot and molten in
the inner parts. The mantle is a semisolid plastic-like consistency,
and the crust is rocky. We know that the inner parts of the Earth are
hot because of the presence of volcanoes.
- Earthquakes also provide important clues about the structure of
the crust because of their location; earthquakes occurs only along
specific fault zones on Earth. Associated with the locations of
earthquakes, we find long surface features such as trenches and
mountains. In fact the earth's crust is broken up into several dozen
observations lead to the theory of plate tectonics: each plate is moving
- Plate tectonics has reshaped what the surface of the Earth has looked
like over time. Our current configuration of continents is not how the Earth's
landmasses have always looked like!
- Plate tectonics accounts for the appearance of many geological features,
e.g., mountain ranges, some volcanoes.
- Plate tectonics is also responsible for some degree of recycling of
the crust of the earth. At locations were plates run into one another, one
plate goes down into the mantle and that crust disappears. At places where
plates move apart from one another, fresh material wells up from the mantle
and new crust is created.
- Plate tectonics is driven by motions in the mantle, which are
probably driven because the inner part of the Earth is hot and molten.
(volcanism) also change the appearance of the surface of the
Earth because they are constantly creating new crustal material. Volcanoes
seen from space.
Most volcanoes are found along plate boundaries, but note that there are
some volcanoes which don't appear on plate boundaries: these are caused
by hot spots under the crust.
- Another driving force on the surface of the earth is
find some sharp mountains and older, more rounded mountains. Erosion arises
both from the action of water and wind. Erosion by water leaves distinctive
markings on Earth as
seen from space
- There is another important thing that changes the surfaces of planets,
though it not so obviously important on Earth: cratering. The solar system,
in addition to containing planets and moons, contains a lot of much smaller
objects ranging from small dust particles to rocky objects miles in diameter;
these are known as meteors. Occasionally, these objects collide with
the planets. When they do, they can make
craters. On Earth,
cratering is not so important because most of these objects burn up as
they pass through the atmosphere; this creates the phenomenon known as
shooting stars. However, the largest objects will make it to Earth's
surface and make craters. In fact, it is possible that the history of
life on Earth has been profoundly affected by a few large meteoric impacts
in the past. Craters on Earth rapidly disappear because they
are covered up by new material (volcanism), destroyed as a plate moves
into the mantle when it collides with another plate, or are
- Not only the surface is changing, but also the atmosphere changes
with time. If you looked at Earth as a planet from space, you'd clearly see the
presence of an atmosphere with cloud patterns that change over time. However,
atmosphere is just a tiny layer of Earth.
- To summarize, we have discussed 4 potentially important physical
processes that affect the surface of Earth, and by inference of other rocky planets:
plate tectonics, volcanism, erosion, and cratering. What do the presence or absence
of these physical processes tell us about the nature of planets, and how might
we determine what to expect on other rocky planets?
- The presence of atmosphere has an important effect: if a
planet does not have an atmosphere, it won't have significant erosion,
and as a result, cratering may become more important (although the
amount of cratering can also be affected by the presence of volcanism
which can erase craters). What determines whether a planet will have an
atmosphere? As previously discussed, it depends on the distance of the
planet from the Sun (which affects its temperature) and the strength
of the planet's gravity.
- The internal temperature of a planet is important: if the internal
temperature isn't hot enough for it to be in a liquid-like form, then it
won't be possible to have volcanism and, likely, plate tectonics. What
determines how hot the inside of a planet will be? We think that
all planets started out hot when they formed, because the energy of
gravitational collapse acts to heat up an object formed by gravity; it
is likely that bigger planets generate more internal heat during their
initial formation, because the stronger gravity leads to more heating.
In addition, bigger planets will take longer to cool off than smaller
ones, so one would expect that bigger planets will have hotter cores
for a longer time than smaller planets.
- Now we can consider the other rocky inner solar system ojbects,
with the question: how similar are they to Earth? Of the dominant forces
which occur on Earth (plate tectonics, volcanism, erosion, cratering),
which occur on the other planets, and in what order of importance? What
do we infer about the nature of these planets? Is it consistent with
what we expect?
- Moon. Not a planet, but useful as a comparison to Earth, especially since
it's at the same distance from the Sun (same temperature), but it's smaller.
Apparent features are
from Earth. Two distinct types of regions: highlands and maria. Dominant
surface feature is
difference in number of craters between highlands and maria; this can be
used to infer relative ages of the two regions. Because of prevalance of
craters, there is clearly a lack of volcanism, erosion - understood given
lack of atmosphere and smaller size of Moon compared with Earth. However,
maria suggest that there was some sort of volcanic processes in the past.
- Mercury. Learned about from Mariner spacecraft in 74-75. The most
prominent feature is craters -
looks a lot like the Moon. One
interesting geological feature are
long cliff like regions.
We do not see any volcanoes or clouds. We infer that Mercury has
no atmosphere and a cool interior. However, we suspect that the interior
of Mercury was once hot: we see cliff-like features, called scarps, which
plausibly formed from shrinking crust as planet cools.
- Venus. Although very similar in size to Earth, it's actually quite
a different place. Venus appears totally
covered by clouds
when viewed from Earth; it has thick atmosphere, mostly CO2 .
Although clouds shroud Venus, the surface was mapped by the
in the early 90's using radar; from this, it is possible
to reconstruct a 3D map of Venus' surface and as a side result to simulate
Overall, the topography is flatter than on Earth:
the main geological features are
rolling plains, highlands, and lowlands.
There are not many craters, but there are a
few big ones;
the lack of small craters is probably because of the thick atmosphere.
There does not seem to be any evidence that the surface of Venus is broken
up into plates. However, Venus does have
(although we have not seen any active ones), and it has some peculiar
circular bulges, called
which appear to have a volcanic origin. The implications are that Venus has a
hot core, though the lack of plate tectonics suggests that the structure of
the mantle/crust may be very different from that of Earth.
Interestingly, erosion does not appear to be a significant process
on Venus, probably because of the lack of liquid water, and low winds.
- Perhaps the most dramatic difference between Earth and Venus is that
Venus has a very high surface temperature (470 C, about 900 F!); this arises
because of an effect called the greenhouse effect.
- What determines the temperature of a planet? To a large extent, it
is determined by the distance of the planet from the Sun. How does this
work? Sunlight is absorbed by the planet causing it to heat up. As the
planet gets warmer it starts to glow with its own blackbody radiation.
The warmer it gets, the more it glows. The process stops when the amount
of glowing from the planet balances the amount of heating from the Sun.
When the details are worked out, one finds that planets farther from the
Sun should be cooler than those closer to the Sun. But the proximity
of Venus to the Sun is nowhere near enough to cause its very high
temperature: simple balance calculations show that Venus should have
a temperature only a bit warmer than Earth, and if one takes account
of the fact that it is covered by clouds, which reflect light well, we
might even expect that it would be a bit cooler than Earth!
- However, note that the kind of light that the planet emits is not the
same kind of light that is incident on it from the Sun. Since the planet
is cooler than the Sun, it glows predominantly at longer wavelengths,
in the infrared part of the spectrum. So incoming sunlight is predominantly
visible light, but outgoing radiation is predominantly infrared light.
- The greenhouse effect can occur when light (energy) is incident on
a planet with an atmosphere. The atmosphere of some planets transmits
some wavelengths of light, but is opaque to other wavelengths because
of the chemical composition of the atmosphere, clouds, etc. On Venus,
sunlight, which is composed largely of visible light, passes through
the atmosphere and warms the planet. The planet, as a solid, dense
object, shines light back into space, but since it is much cooler than
the Sun, this radiation is predominantly infrared light. However, the
infrared light cannot penetrate the dense clouds of carbon dioxide in
Venus' atmosphere, so the energy cannot escape. As a result, Venus is
heated. It keeps getting hotter until the re-radiation of the Sun's
energy is able to escape - but this doesn't occur until Venus is several
hundred degrees hotter than it would have been without an atmosphere!
- The greenhouse effect also occurs on Earth - Earth is warmer than
it should be based on its distance from the Sun, although it is not
nearly so dramatic as for Venus, because our atmosphere has a composition
that is much more transparent to infrared light. There is a serious
concern, however, that the amplitude of the greenhouse effect on Earth
is increasing because human activity (in particular the burning of
fossil fuels, as well as deforestation) is increasing the
amount of carbon dioxide in the Earth's atmosphere by a significant amount.
the potential of raising the Earth's temperature by a few degrees,
which could have some very serious consequences including partial
melting of the polar caps and global climate change. There is already
strong scientific evidence that
global temperatures are increasing;
there is also evidence that
the abundance of greenhouse gasses are increasing;
there is reasonable evidence that human
production of carbon dioxide is the cause of this effect. Although
there is not complete agreement on this, the potential consquences
of global warming are sufficiently serious that it seems prudent to
avoid behavior which is likely to increase the greenhouse effect. We
definitely know that the greenhouse effect is real - all one needs
to do is learn about Venus! Given that fossil fuel resources are
finite in any case, it seems hard to justify not decreasing our
consumption of them. Technology already exists to improve efficiency
of burning of fossil fuels (in particular, in automobiles), and there
are several promising technologies for alternative energy production;
we need to invest in these.
about greenhouse effect on
Earth and climate change
In many ways Mars may be the planet most similar to Earth. It is smaller
and further from the Sun, but still has enough gravity to have a thin
atmosphere. The atmosphere is mostly carbon dioxide like Venus, but it
is thin enough that it doesn't have a runaway greenhouse effect, so the
temperature is not so hot; combined with the fact that it's further from
the Sun than Earth, the temperature is typically a few tens of degrees
cooler than Earth. Mars also has similarities to Earth in that it has
a rotation period close to that of Earth (similar length days), and a tilt
of rotation axis similar to Earth (so there are seasons, although it's
more complicated on Mars because Mars also has a more eccentric orbit).
- View from Earth. Mars has
which are made of some combination of dry ice and water ice. There are
some large volcanoes. There are surface markings. However, occasionally
there are large, and even global, dust storms.
- More detailed view from orbiting satellites: the
global topography is a bit unusual, with one hemisphere significantly
lower than the other(Mars globe). There are several very large
on the surface, although they do not appear to be active; there is also
a large impact basin (Hellas) on the opposite side of the planet which
may be related. There is a
gigantic rift valley,
comparable in depth to the Grand Canyon but several thousand miles long!
There are many
but the surface also shows features that suggest that
water may have flowed
there in the past.
- Overall, the picture is consisent with our expectations: we expect
that Mars had a hot core and volcanism
at some time in the past. Hard to say if there is evidence for plate
- Because of the possible evidence for water in the past on
Mars, it is an interesting place to look to see whether life might have
developed there. NASA has plans for fairly extensive visits to Mars
over the next decade; the first remote landing was made in 1997 by the
spacecraft, with its rover, Sojourner, which went
roving around the surface to look at specific
rovers were sent to Mars in January 2004 to roam around two different
locations on the planet
(Spirit landing site). There's lots of images and animations available about Mars; see,
e.g. NASA site with lots of images/animatation).
Rover animation from ``Roving Mars''
- We are in an active period of Mars exploration, with several
missions currently at Mars and several more planned in the next decade or so;
are there right now!
- Much of the interest in Mars stems from the possibility that water
has existed in liquid form at some point in the past on Mars. The existence
of liquid water is thought to be critical for the possible development of
life. The Viking landers on Mars in the 70s looked for evidence of life,
but didn't find anything conclusive.
- Additional interest in the possibility of life on Mars was spurred
by investigations in the 90s of a meteorite
found in Antarctica in the 80s. This meteorite has been determined to
have originally come from Mars. The composition of some of the compounds
in this meteorite, as well as microscopic features in the meteorite,
suggest to some people that there may have been life on Mars at some
point in the past.
- Rovers currently on Mars have found minerals that almost conclusively
suggest that water existed on Mars at some point. Still no unambiguous signs
of life, however.
- Brief general discussion of missions to planets, manned and unmanned:
scientific benefits, human motivations, cost, etc.
- The outer planets
are much larger than the inner planets. Because of
their large mass and cooler temperature, they have a very different composition;
they are mostly made of light elements with extended atmospheres in a
- Outer planets exist in much bigger systems than inner planets.
All outer planets have ring systems and moons.
- In some respects, moons are similar to terrestrial planets. However,
they have somewhat different compositions: more icy, which is understandable
given that they formed outside the frost line. The moons of the outer planets
are similar in size
to the Earth's moon, so it was expected that they would look similar.
However, there is a
of geologic features on different moons of the outer planets!
This is most dramatically seen in Io, the inner
moon of Jupiter; it has
on its surface, even though it is a relatively small moon. Europa is of
particular interest right now, as the surface suggests the appearance
of a frozen liquid. Titan, a moon of Saturn, is also of great interest;
as the largest moon in the Solar System, it has an atmosphere!
- Some of these moons have an important physical process that doesn't
apply to the inner planets: tidal heating. The
moons of the giant planets, especially those closest to the planet,
are constantly being pulled by gravity by an amount which differs from
one side of the planet to the other, leading to heating of the internal
parts of the moons.
- The same tidal forces - the difference in pull of gravity from one side
of an object to the other - are responsible for the ocean tides that we
have on Earth
All of the outer planets have rings, although
the rings of Saturn are by far the most dramatic.
- What are they? Rings are made of many small particles.
- Where to they come from? All of the rings exist fairly close
to the parent planet, closer than the moons. They may be the result of previous
collisions, especially with some of the smaller moons, and tidal forces.
We don't know how long rings live around a planet: maybe we're lucky to
be seeing Saturn's rings as they are now?
- Why do they have so much structure? The dynamics of rings are very complex; they are
by the gravitational force of moons nearby the rings.
- Since they are made of gas, what we are seeing when we
the outer planets are clouds. When we look at
these planets, we see clouds in the outer layers. We observe different
colors when we observe to different depths, because the clouds that form
at different heights in the atmosphere are made of different compositions.
- All outer planets rotate very fast, so
very important in determining what they look like; also, storm systems.
- Our understanding of the interiors of the outer planets comes
mostly from our physics models of how gas balls work. We expect that
the pressure increases as one moves in towards the centers of these
planets. Eventually, you reach a layer far inside of the planet where
the pressure is high enough that the hydrogen takes a liquid form. There
is probably a small rocky core in the centers of the outer planets, but we
are not sure.
- The large mass of the outer planets makes the interiors hot when
they form, even more so than for the inner planets, because the larger
gravity speed up the atoms more as the planet is formed. The hot cores
of the outer planets lead to significant heating from the interior.
Since heat escapes from the inside of these planets by the motion of
hot material upward, we get belts (dark material falling down) and zones
(bright material coming up).
- Magnetic fields may be important in these systems.
- In detail, all of the outer planets differ from each other. Jupiter
is the most colorful, with lots of atmospheric structure. Saturn has
less atmospheric features, and has the largest ring system of all of the
outer planets. Uranus is mostly featureless, but Neptune has cloud features.
Differences probably arise from differences in temperature, rotation, etc.
- Much of what we know about the outer planets was learned from the
Voyager missions, which flew by all of the outer planets between the late
70's and late 80's. The Galileo spacecraft recently orbited Jupiter for
several years, studying it and its moons in more detail. The Cassini
spacecraft is on its way to study Saturn and its moons.
- What is the age of the Solar System?
- We can determine ages of certain elements because many elements
have an intrinsic property called radioactivity, which causes an element
to change to another type of element as time passes. Most elements are
very stable, but some elements change faster. The powerful thing about
radioactive decay is that the time which it takes for a certain amount of
an element to change to another element is very repeatable. Consequently,
by measuring the fraction of an element in some specimen which has
decayed, we can accurately tell how long the specimen has been around.
- When we do this for Earth rocks, we find there are some rocks
which are as old as 3.2 billion years. When we do it for Moon rocks,
we find the oldest rocks are about 4.2 billion years old. When we do it
for asteroids, we find some rocks as old as 4.6 billion years old.
- We thus infer that the the Solar System is at least 4.6 billion
- We are in an exciting era because in the past few years, we have
actually started to discover other stars which have planets around them.
This is exciting for several reasons: 1) until now, we didn't know for sure
whether other planets existed, although we expected that they did, 2)
the presence of other planetary systems allows us to test our theories
about how our Solar System formed. Do other planetary systems have similar
characteristic to ours, or are they different?
- Unfortunately, it is very difficult to discover planets around
other stars. Seeing them directly is very difficult because they are
much fainter than the stars around which they are orbiting, and especially
because we cannot get sharp enough images to distinguish a planet from
a star; images are blurred out because of the effects of the Earth's
atmosphere, and even for telescopes above the atmosphere, by basic limitations
of a telescope and the nature of light.
- Despite this, several hundred other planetary systems have been discovered
in the last decade or so.
The planets in these systems have not been seen directly, instead,
their presence has been inferred from their gravitational effect on the
star around which they orbit. When planets orbit a star, they also cause
the star to move slowly in a very small orbit itself; the orbit of the
star gets larger for planets with more mass, and for planets that are
closer to the star. It is possible to detect the
motion of the parent star through the Doppler effect. The Doppler
effect causes the light from a star to shift to a slightly different
wavelength as the star alternately moves towards and away from us in its
tiny orbit. By measuring the motion of the star, we can estimate the mass
of planets around the star, and the radius of the orbits in which they
- Interestingly, it appears that
other planetary systems
are quite different from our own (see also
http://exoplanets.org/massradiiframe.html. Several of these massive planets
are found very close to the parent star, unlike the situation in our
Solar System. Also, several of the planetary orbits are very eccentric
ellipses, unlike the nearly circular orbits found in our Solar System
- However, before jumping to the conclusion that our Solar System
is an unusal place, one has to realize that this technique for detecting
planets is more sensitive to higher mass planets than to lower
mass ones, and more sensitive to planets that are closer to the stars than
to ones that are further (because the more massive and closer planets cause
a larger motion in the star). Consequently, so although only planets with masses
comparable or larger than Jupiter's mass have been discovered, this does
not mean that there are not Earth-like planets out there!
- Even still, according to our model of Solar System formation, we
would have expected it to be difficult to form massive planets close to
a parent star, since it should be too hot there for light elements to
be bound to a planet when it is forming. As a result, it would be very
nice to know whether these massive, close-in extrasolar planets are
gaseous or not. Amazingly, we can get an estimate of this for a handful
of the objects. We do this for extrasolar planets that happen to pass
in front of their parent star, leading to a slight dimming of the starlight.
This dimming can be used to determine the size of the planet, which,
combined with the mass estimate from the orbit, can be used to
calculate the mean density of the object. For the few objects for which
this has been done, we find that they are comparable to the Jovian,
gaseous, planets in the solar system; the general feeling is that all of the
very massive planets such as the ones being discovered must be gaseous objects,
as it's unlikely to get such large concentrations of the relatively
rare heavier elements.
- These discoveries have led astronomers to reconsider the applicability
of the nebular model to all Solar Systems. Before we actually found other
planetary systems, we would have predicted that massive planets could not
exist close to the parent star; but recent discoveries show that they do!
Consequently, our models may need to be revised. However, another possibility
is that these planets formed farther away from their parent stars, and
have migrated closer to the stars, although the details of how this might
happen are not totally clear. This is a good example of how
science works - we develop models for how things work, and these often
turn out to be wrong once we collect new information! Science is an ongoing
- People are actively working to devise systems to detect larger
numbers of planets, and planets of significantly lower mass towards the
Earth range. There also exists the possibility of getting more detailed
measurements of atmospheric abundances in these planets through transit
spectroscopy. These studies are opening up a whole new field...