The Birthplace of Stars The space between the stars is not completely empty. Thin clouds of hydrogen and helium, seeded with the “dust” from dying stars, form in interstellar space.

Dark Clouds gather

Molecular Clouds Sometimes (especially in spiral arms), the gas is compressed enough that the dust is thick and gravity can collapse knots in these “molecular” clouds to make new stars.

Where Stars Begin

Hot Stars make their Nurseries glow The Orion Region Infrared Optical

A little bit of spin goes a long way… Galactic shear and turbulence give every core a little spin (once round in 10 million years). But they get a lot smaller, and the spin goes up – to orbital! It is for this reason that we believe there are many planetary systems – it is part and parcel of the star formation process to make a disk. Typical Galactic spin makes disks about the size of our Solar System…

Half the time, two (or more) stars form Both observations and computer simulations show this, and it must be Nature’s way of dealing with excess angular momentum, but we don’t understand the details of how or why multiple systems form.

Star Formation is beautiful, but ephemeral Within about 10 million years, the birth-cloud is shredded, and the disks are dissipated. The process of starbirth has ended.

The Sword of Orion The nearest great stellar nursery to us is the great Orion molecular cloud which is about 1000 light years away, and manufacturing thousands of stars. This is probably how the typical star is made.

The glowing tip of a molecular “cigar” The Orion nebula is powered by 4 high mass luminous stars, which have cleared out their birthplace and are eating at a long cloud pointed at us. The Trapezium

Near them, lower mass stars are forming Hubble Space Telescope

They look like little windsocks The blast from the luminous stars is eating away at the little guys

The heart of them contains a potential new solar system “Proplyds” are new star-disk systems

Orion Proplyds

The Star-Disk system forms… Disks are a natural and necessary part of star formation, and essential for planet formation.

Emitting bipolar jets… The jets are powered by the stellar magnetic field and rotation, and takes away excess angular momentum

Which can extend for many light years negative image

The jet shocks are called Herbig-Haro Objects

In the center - T Tauri Stars The new young star is exposed, while the accretion disk is still in place. The spectrum of all is seen together (so we don’t have to image disks in order to know they are there).

The Young Star is Very Active The star itself is 3 times bigger. The magnetic flux is hundreds of times stronger than the Sun, and huge starspots and strong X- rays are seen. Particle fluxes are also much stronger and more energetic, and can account for some strange isotopes in meteorites.

T Tauri Stars might make Chondrules One theory, not fully accepted, says that the magnetospheric accretion and wind could be the source of the ubiquitous marble-sized nodules in meteorites, which must have been suddenly melted, then cooled in a few hours…

The stage is set for planet formation

Cosmogony – formation of planetesimals As if by magic…

The central mystery is how the dust makes rocks It is easy to make snowballs, but hard to make sandballs. Colliding meter-sized rocks together is more likely to break them apart than build a bigger rock.

We see rocky and icy planetesimals even today Asteroid Eros Comet Tempel Comet Halley

Cosmogony – terrestrial and gas giant zones The composition of the disk around the Sun depends on distance from it, through temperature (can you have ice or not). Since icy material is plentiful, you can make big planets in the outer reaches. Once big enough, they can grab gas from the disk (more plentiful).

We see remnant disks around other stars These “debris disks” are dust seen in scattered light. The dust would have to be pretty recently produced (by collisions among larger bodies), since the Poynting-Robertson effect causes it to spiral into the star in less than a million years. Warps and rings may be caused by planets.

Compositions of the Planets

Planet Formation : Rocky and Gas Giants Planetesimals agglomerate into protoplanets, then planets. If the core is large enough early enough, gas from the disk can be captured. This is more likely in the outer solar system. movie

Violent Accumulation of Rocky Planets

Core Accretion Model of Giant Planet Formation In the standard model, the planet starts like a core of rock/ice material. When the zone is depleted, things slow down, while gas is also slowly accreted. Once the mass of gas is nearly that of the solids, the gas accretion rate suddenly runs away, creating a gas giant planet. This models tends to take too long, but refinements like migration, disk formation around the planet, and better treatment of opacity in the planet’s envelope can speed things up considerably. Accretion rates, solid and gas (dotted). Total mass accumulated, solid and gas (dotted).

Orbits and tilts of the Planets

Problems Posed by Extrasolar Planets 1)If the planets are formed in a disk, why don’t they have circular orbits? 2)How did gas giants get to be so close to the star? Lynette Cook One possible answer: orbital perturbation and migration. Planets may not now be in the orbits they formed in. They can interact with the disk to move in, or can disturb each others orbits (chaos).

Orbital Migration : Planet-disk interactions A terrestrial planet (lunar to 10 Earth mass) will raise waves in the disk which take angular momentum from the planet. It spirals towards the star – alarmingly fast! If the planet is Jovian, it opens a tidal gap in the disk, and then shares whatever fate the disk does (accreting to the star or drifting away if close to the outside).

Do we need planetesimals for gas giants? Perhaps we can make giant planets directly from the disk. If the disk is gravitationally unstable (cold and massive enough), then direct instabilities can collapse a whole region of the disk, leading directly to a gas giant planet in a very short time. You’d have to account for metallicity enhancements in our planets, and the fact that most exoplanets are in metal-rich systems. Also, we don’t seem to actually observe unstable disks. But that is also how you make brown dwarfs or binary stars… This obviously sometimes happens, but gets us back to the question of what is a planet?

There is still a lot to be learned about Planet Formation And our tools are getting better all the time!