Extra-Solar Planets

Topics in this lecture:•Review of planet formation•The Habitable Zone•Basic properties of discovered exoplanets•Hot Jupiters•Super-Earths

Exoplanet - A large body orbiting a star other than the Sun.

How to make a planet

• Large, cool cloud of gas and dust– Gas makes the star, dust is necessary for planet

formation– Dust is usually made of metals (Fe, Ni, Al), rocks

(silicates) and ices (solid H2O, CH4, NH3)

– Mostly H and He (these two elements make up about 98% of our Solar System)

• Cloud begins to collapse under its own self-gravity

How to make a planet

• Collapse causes– Increase in temperature (conservation of energy)– Increase in rate of rotation (conservation of

angular momentum)• Result is a rapidly rotating disk of gas and dust– Again, dust means rocks, metals and ices– at sufficient distance from the parent star,

hydrogen compounds (H2O, CH4 NH3) are important in formation of giants (Jovian planets are far from the Sun)

How to make a planet

• Accretion– Dust grains collide to form larger particles -->– Collide small particles to form still larger particles

--> – Collide large “boulders” to form planetesimals

(asteroids, comets, Kuiper Belt Objects)--> – collide planetesimals to form planets

How to make a planet

Nebular Hypothesisa) Solar Nebulab)Contraction into rotating disk w/ hot centerc) Dust grains accrete to form larger and larger

particlesd)Large particles sweep out more material to form

planetesimalse)Planetesimals eventually collide to form planets

The Habitable Zone

The region around a star within which the requisite conditions for the existence of life can be met.

Typically these conditions include:– Suitable temperature for liquid water– Existence of water itself– Appropriate cosmo-chemical composition– Are there other conditions or different ones? Are

these necessary and sufficient conditions for life?

The Habitable Zone

Depends on:• Distance to parent star• Type of parent star• ATMOSPHERE– Albedo– Greenhouse effect– Dynamics (=atmospheric motions)

Overview of ExoplanetsPlanet (IAU definitions of Planet, Dwarf Planet and Small Solar System Bodies)

(1) A "planet” is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit.

(1) A "dwarf planet" is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighborhood around its orbit, and (d) is not a satellite.

(1) All other objects except satellites orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

Overview of ExoplanetsExtrasolar Planet (IAU)

Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars or stellar remnants are "planets" (no matter how they formed). The minimum mass/size required for an extrasolar object to be considered a planet should be the same as that used in our solar system.1. Substellar objects with true masses above the limiting mass for

thermonuclear fusion of deuterium are "brown dwarfs", no matter how they formed nor where they are located.

2. Free-floating objects in young star clusters with masses below the limiting mass for thermonuclear fusion of deuterium are not "planets", but are "sub-brown dwarfs" (or whatever name is most appropriate).

Overview of Exoplanets

Discovery• First discovered extrasolar planet: 1988• Number to date: 405• Largest: 25 MJ

• Smallest: 0.006 MJ (7x10-5 MJ possible comet)• Closest to parent star: 0.0172 AU• Furthest from parent star: 670 AU• Detection methods: numerous

Planet Detection MethodsRadial Velocity – doppler shift in spectral lines of

parent starA two-body system actually orbits the common barycenter (center of mass). As the star approaches (recedes) the light from the star is blue-(red-)shifted. The frequency and velocity can determine the mass of the orbiting body, in this case a cool, dim planet.

Scatter plot of mass, m, and semimajor axis, a, for exoplanet discoveries through 2009-11-09, indicating the discovery method using distinct colors (radial velocity = dark blue, transit = dark green, timing = dark purple, astrometry = dark yellow, direct imaging = dark red, microlensing = dark orange, pulsar timing = purple), with Solar System planets indicted for reference (in white with gray outlines).

Hot Jupiters

• Large planets, orbiting close to parent star• Too close to be in the Habitable Zone• Still some interesting atmospheric chemistry

and physics– Silicate rain?– Tidal locking causing interesting atmospheric

waves?– All atmospheric chemistry/physics is still

speculative

Hot Jupiters1. They have a much greater chance of transiting their star as seen from a further

outlying point than planets of the same mass in larger orbits. 2. Due to high levels of insolation they are of a lower density than they would

otherwise be. 3. They are all thought to have migrated to their present positions because there

would not have been enough material so close to the star for a planet of that mass to have formed in situ.

4. They all have low eccentricities. This is because their orbits have been circularized by the process of libration. This also causes the planet to synchronize its rotation and orbital periods, so it always presents the same face to its parent star - the planet becomes tidally locked to the star.

Hot Jupiter systems can still have terrestrial planets. In computer simulations, as the hot Jupiter migrates inward, it scatters material (planetesimals, etc.) outward. Since the inner stellar system material is mixed with material outside the “frost line”, these terrestrial planets would be water-rich.

Super Earths

• Terrestrial (“rocky”) planet• More massive than Earth, less massive than

Jupiter (ranges used in the literature range from 1 ME – 10 ME)

• Not necessarily “habitable”, as the name might suggest

• Host stars are metal-poor• Some super-Earths have been found in the

habitable zone around main sequence stars

Good Examples of Stellar Systems

• Gliese 876a) Parent star:

– M3.5 V, T=3480 K, L=0.0124 Lsun

b) Gas Giant– M=2MJ, a=0.208 AU

c) Gas Giant– M=0.62 MJ, within orbit of Gliese 876 b

d) Super Earth– Inside orbits of both Gliese 876 b and c

The Gas Giants in this system are within the Habitable Zone, the Super Earth is too close (hot) for liquid water.

Good Examples of Stellar Systems

• Gliese 581a) Parent star:

– M3 V, T=3480 K, L=0.013 Lsun

b) Gas Giant (Neptune-sized)– M=16 ME, a = 0.04 AU

c) Rocky Planet– M=5 ME, a = 0.07 AU, within habitable zone (Temperature could

be as low as -3 oC or as high as 500 oC, due to runaway greenhouse akin to Venus)

• Super Earth– M=7 ME, a = 0.22 AU, within habitable zone

a) Super Earth– M=1.9 ME, a=0.03 AU