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4.1 Exoplanets [Astronomy: State of the Art]


The detection of planets around other stars
is one of the most exciting discoveries in all of science in the last few decades. Starting
in 1995, a new field has opened up as we found planets around other stars—starting with
jupiter and super jupiter mass, and moving towards earth mass planets, and terrestrial planets.
Who could have imagined, 20 years ago, that around a system in the constellation cancer,
we might visualize and eventually detect, multiple planets—gas giants in solar system
type orbits around a nearby star. (Music) A plot of the discovery of planets in these last 20 years around other stars gives a sense
of the dramatic progress. This is a logarithmic scale of mass and the limit on mass detection
for planets has marched steadily downwards from the initial discoveries, which were
jupiter mass or larger, to reach earth mass just in the last few years. The last couple of
years has been overwhelmed by discoveries from the Kepler telescope in space, launched
by NASA. In addition to the steady march of a downward mass limit on the lowest mass planet
detected, The numbers have continued to accumulate, with a doubling time in the number of exoplanets of between 18 and 24 months. Extrasolar planets, or exoplanets, are one of the most hot topics in astronomy
at the moment. The first discoveries puzzled astronomers enormously because rather than
finding giant planets in typical giant planet orbits, they found “hot jupiters” on incredibly
tight, hot orbits of their parent sun-like stars. The very first exoplanet discovered,
51 peg, is on a 4.4 day orbit around its sun-like star. Think, Mercury takes 88 days to orbit
the sun, So this is a Jupiter mass planet far closer to its star than Mercury is to
the Sun. Many of the early discoveries were like this, so called “hot Jupiters” and astrophysicists
were confused because they had no way of explaining how giant planets could have possibly form
so close to their parent stars. Even with the 100 or so detections coming in, in the late
1990s, it was clear that there were lower mass planets ready to be found. The distribution
of masses of the planets found at any point in the last the last few decades, rises rapidly towards the detection limit. thats always a sign that there are low mass planets lurking just below the detection
limit. Imagine you are a fisherman and had a net with 1 inch holes, and your net was
filled with fish- maybe a few who were a few feet long, many that were a foot long, and
a large number 3 or 4 inches, bigger than the hole size of your net. The rising number
of fish towards the size of holes in your net is an indication that there are indeed many
fish to be found that are just too small for your net to catch. Astronomers also needed
better nets to net the small planets that are out there. As of early 2013, there are
over 3000 exoplanets known. Many of these remain to be confirmed, but in all likelihood
they will be, because Kepler detections have a 90% probability of being confirmed once they
have sufficient data. Many of these are Jupiter mass and most are more massive than Uranus
and Neptune, but gradually the detection limit has reached towards super Earths and Earths
and there’s every expectation that a very large number, perhaps the majority of exoplanets are
small, rocky, terrestrial planets. The presence of so many giant planets, so close close to
their stars forces a revision of our idea of how the solar system formed (and other
solar systems), because there is no way for planets to form that large, that close. There’s
not enough material, and there’s no way for that material to condense and go into an object
in a stable orbit. The early exoplanet discoveries had other puzzles too. When people plotted out the eccentricity of the orbits, or the degree with which they deviate from circularity, these planets were in much more elliptical or eccentric orbits than the
planets in the solar system. In fact, most of the first few hundred exoplanets had eccentricities of 10 or 20% which is rare in the solar system. Nobody could explain why the orbits were so
eccentric. The excitement of discovering exoplanets was tinged with frustration because people
could not explain how these planets formed. It seems now, that planets can migrate and
that planetary migration has to be a standard part of the way solar systems work.
Not only that, but planets go through periods in the history of a solar system where their
orbit are unstable. They can change places, move inward or even outward, and occasionally,
get ejected from the system entirely. Computer models have been essential in helping us
understand the chaotic motions and orbits of solar systems. These new models, applied
to our solar system, suggest that the giant planets have not always been in the positions they
hold now. In fact, there’s indirect evidence that Uranus and Neptune swapped places early
in the history of the solar system and that this rearrangement was associated with a perturbation
to the comet cloud that produced a spike in the impacts on the earth and the terrestrial
planets. If true, this solves a mystery of several
decades standing whereby theres an era of heavy bombardment about 3.8 to 3.9 billion
years ago—present in the cratering record on the moon and Mars and Mercury. There needs
to be a reason to explain this heavy bombardment because in general, after a solar system forms,
the amount of heavy, rocky debris objects diminishes with time as they form planets
and are swept away. there should be a continuous decrease in the cratering rate. Instead we
see a spike 3.8 to 3.9 billion years ago; presumably impacting the earth too, but the
cratering record is not good on this planet because of the erosion and tectonic activity. In some
ways we can think of the way planets behave as a game of cosmic billiards. A very violent
history early on, as the planets formed, and then occasional violence later on, as the
planetary systems go through instabilities. Some of these instabilities are triggered
by resonances, such as when giant planets enter orbital states where the ratio of their
periods is a whole number- 2:1 or 3:1 or 3:2. These resonances have been observed in the
moons of giant planet systems, but they could apply to the giant planets themselves. Planetary
migration is now a standard part of the theory of how solar systems form. Put simply, it
is impossible to form a hot jupiter in its place. It must have formed further out and
migrated in. While migrating in, it might fall into the parent star or park in an orbit
and form a stabilized facing position, tidally locked to the parent star. No one was present
when these events happened, so it’s very hard to verify these models. We have to look for
subtle clues in the current placement of the planets and in the behavior of the comets,
asteroids, and meteors. Another way to think of this is the fact that the solar system is
essentially ‘full’ in dynamical terms. We can visualize the solar system and we know
that the space between planets is vastly larger than the size of the planets themselves. The
solar
system indeed seems like mostly empty space

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