Migrating Planets! 7 April 2016

Origin of the Solar System 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 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 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 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.