Migrating Planets!
7 April 2016
Origin of the Solar System
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Elements were made inside stars
Stars formed from giant molecular clouds
Planets form in a disk around the protostar
Giant planets form from accretion onto cores
Terrestrial planets are only the cores
The giant planets form far away
Terrestrial planets form close to the star
Characteristics of our Solar System
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Regular planet orbits
Planets are closely spaced
Terrestrial and Jovian planets
Asteroids and comets leftover
Satellites and rings imitate a miniature Solar
System
Stages in planet formation
• Dust collects in the mid-plane of the
protoplanetary disk
• Grows by mutual collisions: ‘accretion’
• Planetesimals (about 1km across) grow and
collide
• Giant collisions are the final stage
• Giant planet cores are bigger outside the
frost-line: they attract gas to become gas
giants like Jupiter
• Star ignites: the gas and dust blown away
Leftovers
• Near the Sun (inside frost-line, also known as
snow line or ice line) rocky objects become
asteroids
• Far from the Sun (outside frost-line) icy
objects form the comets and Kuiper Belt
Objects
• The first exoplanets discovered were labelled ‘Hot
Jupiters’. These planets, although similar in radius to
Jupiter, orbit their stars so close that they are tidally
locked in place with one side in permanent daylight
and the other in perpetual darkness. The close
proximity to their star means it can get incredibly
hot. The hottest thus far is WASP-12b with a dayside
temperature of around 2,500°C that is hotter than
some stars. This orbital distance also means that a
year on these worlds is just a matter of days allowing
us to take multiple measurements of the planet and
the star over short periods of time
Exoplanets: Hot Jupiters
• "Hot Jupiters" — massive planets that orbit very
close to their parent stars — have been found to be
abundant outside Earth's solar system.
• But how did these gas-giant planets in other solar
systems get so close to their parent stars?
• Because Earth's solar system does not host any hot
Jupiters (the giant planets are further out, and
smaller planets orbit closer to the star), scientists
have to rely on observations of distant planetary
systems to solve this mystery.
Where Hot Jupiter’s Form
• Hot Jupiters are similar to planets such as
Jupiter and Saturn.
• These exotic worlds form relatively far away
from their stars, as our solar system's gas
giants did, but then move inward over time in
a process that remains poorly understood
• We're fortunate that Saturn and Jupiter didn't
do this, for such migrations can boot smaller,
rocky planets out of the way, sometimes
ejecting them out of the solar system entirely
But how do they get close?
• Consider Saturn’s rings, that resemble a
protoplanetary disk
• Moons in and near the rings create waves
• Motion of the waves changes the rings and
moon’s orbits (Newton’s Third Law strikes
again!)
• The moons move (migrate)
• The same can happen to a planet, moving
closer to the star
Density waves and
propellers in a
forming disk
Pan closeup
Propeller, but moon causing it still unseen
Planet Migration
• Density waves and gaps allow angular
momentum transfers
• These move the planets (like moons within
rings) around
• Some end up in the proto-star! Some stop, but
why?
• This infant mortality removes planets, like
musical chairs…
• What we see now are just the survivors. Was
Earth lucky?
Summary: Exoplanet facts
• The known exoplanets fall along a range of
sizes, masses, and orbital positions. Sizes and
masses range from smaller and less massive
than Earth to super-Jupiter types of worlds.
Orbital positions range from very close to the
parent star to very distant.
• The region around a star where liquid water
could exist on the surface of a solid planet is
called the habitable zone. Worlds orbiting in
that zone are considered to be prime
candidates where life could be supported.
• More than 22 percent of Sun-like stars have
Earth-sized planets in their habitable zones.
These are important places to concentrate a
search for possible life-bearing worlds.
• The Kepler Mission was launched to search
out distant worlds. It continues its search
today.
Famous Exoplanets
• 51 Pegasi b: The first planet around a star like the
Sun. Astronomers found it using the Observatoire de
Haute-Provence in France, a ground-based facility.
This planet is also known as a “hot Jupiter” because
it appears to be a very warm gas-giant-type world.
• Kepler 186-f: the first Earth-size planet circling in the
habitable zone of its star. Found by the Kepler
Mission in 2014.
• Kepler 11-f: orbits a Sun-like star and has at least 2.3
times the mass of Earth. It may be a gas dwarf, due
to its low density and possible hydrogen-helium
atmosphere.