In a dense molecular cloud far, far away

M16: The Eagle M16: The Eagle NebulaNebula

Dense Molecular gas

Hot, ionized gas

Young stars

Bright star

Ionizing radiation

Dust grain absorbs visible/UV, re-radiates infrared,which escapes

Ion

izat

ion

Fro

nt

IonizedGas

Dusty Molecular Gas

“Evaporating”molecular gas

IonizedGas

Molecules and dust1. Allow efficient cooling of

gas2. “Self-shield” from ionizing

radiation3. Act as a “Refrigerator”

Excited molecule releases energy

Locally denser clouds survive, cool

Distinct Protostellar Nebula

1000 AU

• proposed originally by Kant, Laplace, and others in the 1700's

• solar system formed from a nebula (cloud of interstellar gas) that evolved into a disk.

– initial nebula presumably looked like the molecular cloud cores in Galaxy M 16,.

• Properties of the pre-Solar Nebula– Low density---102cm-3. Compare this to the density of air, 2x1019 cm-3.

– The minimum mass is a few times the mass of the sun.

– The material was well mixed (“homogeneous”).

– Solid material included interstellar grains (“dust”), nebular condensates, and diamonds.

– Chemical composition:• H and He 98%

• C, N, O 1.33%

• Ne 0.17%

• Mg, Al, Si, S, Ca, Fe, Ni 0.365%

– Initially Low Temperature---probably around 50-100K (-200C).

• Numerous regions of gas and dust dispersed in our galaxy

• Largest are known as giant molecular clouds and contain enough gas to make 100000 Sun masses

• Closest large molecular cloud is the Orion nebula (1500 light years away)

• Most of gas is H and He, but about one in every thousand atoms is heavier than He

• Virtually all atoms are combined in molecules (eg H2) (due to low T and relatively high density of atoms: densest areas have about 107 atoms per cm3)

• Chemistry, density and temperature distribution of clouds is complex and varies between and within clouds

• Molecules detected inlcude H2, CO

• In coldest parts of clouds most of the molecules are bound in rocky-icy grains.

• Newly formed stars are known as T-Tauri stars (very bright)

•Note ring-like structures withingas clouds (nebulae), surroundinga central proto-star (here in Orion Nebula)

•Stars older than a few Ma don’t have proplyds, hence planets must form “rapidly” following proto-star formation. Gas giants may have formed quicker than terrestrial planets

•Planet formation in discs notactually observed, hence “circumstellar disks” is a better phrase

•Gaps in disks may indicatepresence of planet

•Note planets may also form around old stars (eg binary systems were one has released dust and gas, that is captured by its companion, some planets also found around single red giants)

• Beta Pectoris– ~50 light years from sun– Disk is 500 AU across– ~100 million years old

• Condensation – slow growth of grains, atom by atom by random collision– Like snowflakes in a snow cloud

Dust grain,1 micron

Atom, 0.0001 micronVery slowly…

• Accretion – Somewhat faster sticking of grains– Condensation makes grains chemically sticky– Friction generates static electricity

Dust grain,10 microns – 1 cm

Dust grain,10 microns – 1 cm

• The condensates take the form of (1 micron size) dust grains in the solar disk.

• These grains will settle to the disk midplane since they are heavier than the H and He gas. What happens next is uncertain. – One possibility is that the thin disk of dust is gravitationally

unstable, leading to the formation of roughly 1 kilometer size objects known as planetesimals.

– Another possibility is that the flow in the disk is turbulent, so that the dust cannot settle out and form an unstable thin disk. In this picture the dust grains collide with each other and stick to form slightly larger bodies, which in turn collide to form yet larger bodies. This picture suffers from the difficulty that bodies between the size of dust and planetesimals suffer the effects of drag, and so tend to spiral into the sun.

Cluster of planetesimalsCluster of planetesimals

• Now sufficiently large that turbulent gas motions don’t blow them in, out, ‘round and about

• Gravity takes over– Planetesimals free to sink into middle plane of

disc– Planetesimals gravitationally attract each

other

• Clusters of planetesimals become self-gravitating

• Collisions are usually soft, since everything is co-rotating together

• Soft “soil” on surface allowed more “sticking”

Evolutionary Differentiation– Denser materials

(iron, nickel) in center– Lighter materials

(silicates) toward surface

• Radioactivity heats, melts protoplanet, allowing differentiation

“In situ stratification”– The first

planetesimals are mostly metals, while solar nebula is hot

– As nebula cools, lighter elements for planetesimals

– Melting, differentiation during formation

• Once the larger of these particles get big enough to have a nontrivial gravity, their growth accelerates. – Their gravity pulls in more,

smaller particles, and very quickly, the large objects have accumulated all of the solid matter close to their own orbit.

– The accretion of these "planetesimals" is believed to take a few hundred thousand to about twenty million years

• A wind of charged particles from star(s) now acts to erode disk from inside out (solar wind is a remnant of this).

• In solar system gas giants may have formed at this time, before most of gas of disk was blown away.

• Interaction of Jupiter and Saturn kicked out planetesimals to form Oort Cloud

• Interaction of Uranus and Neptune kicked out planetesimals to form Kuiper belt

• Formation of Condensates and differentiation– The solar nebula was originally

gas, – as the density of the gas

increased solid material began to condense out.

– The process is the inverse of sublimation, in which a solid such as ice goes directly to the gas phase (water vapor in this example).

– A solid formed by condensation is called a condensate.

Differentiation

RadialPosition

Temperature(K)

Dominant Solid

1. 1700 Refractory minerals, (CaO, Al2O3 TiO)

2. 1470 Metals (Fe, Ni, Co, and their alloys)

3. 1450 Magnesium rich silicates

4. 1000Alkali feldspars (silicates abundant in alkali elements (Na, K, Rb)

5. 700 Iron sulfide FeS (triolite)

6. 400 Fe condenses

7. ~350 Hydrated minerals rich in calcium

8. ~300Hydrated minerals rich in Iron and Magnesium

9. 273 Water ice

10. 150 Other ices (NH3, H2O, etc)

Cold Homogeneous Accretion model– Terrestrial planets accreted as a

homogeneous masses of disk material

– Later differentiated by internal heating– Heat supplied by:

• A) Accretionary Heating– Meteorite bombardment

• B) Core Formation and the Heat of Differentiation.

– Gravitational collapse and release of heat energy

• C) Radiogenic Heating– Decay of radioactive

elements

Hot Heterogeneous Accretion– Most scientists today prefer a model

where large chunks of material, some of which were metal + silicate, others predifferentiated as one or the other, violently coalesce to form the Earth.

– Metal sinks to the core due to negative buoyancy (silicate is hot enough to be plastic and "squishy" if not actually molten).

– Conditions are HOT – volatile elements were lost to a significant degree.

– Based on:• Asteroids

– "unassembled" planets,

– already differentiated into chemically different types.

– Therefore, Protoplanetary material was already differentiated.

• Many other planetary systems have been discovered within the past 18 years.

• Cannot be imaged directly, since too far

• Indirect evidence – Analysis of light from parent

star• Wobbling of star due to

mass of planets causes Doppler shift

• Most extrasolar planets are Large (>1 Jupiter mass)

• First extrasolar planet confirmed• announced in late 1995 by astronomers studying

51 Pegasi, a spectral type G2-3 V main-sequence star 42 light-years from Earth.

• High-resolution spectrograph found that the star's line-of-sight velocity changes by some 70 meters per second every 4.2 days (a doppler shift).

• planet lies only 7 million kilometers from 51 Pegasi

– much closer than Mercury is to the Sun • planet has a mass at least half that of Jupiter. • temperature of about 1,000 degrees Celsius

– Probably lacking an atmosphere, – planet may be a nearly molten ball of iron

and rock with seven times the Earth's diameter and seven times its surface gravity.

• One side may permanently face the star, much as the Moon's does the Earth

3 planet system– Planet 1:

• 0.7 Jupiter masses• 0.06 AU orbit

– Planet 2:• 2.1 Jupiter masses• 0.83 AU orbit

– Planet 3:• 4.3 Jupiter masses• 2.6 AU orbit

• At last a planet has been confirmed with an orbit comparable to Jupiter's; – a distance of 5.5 AU from the star,

(Jupiter's is 5.2 AU). – the 13 year orbit is slightly elliptical

rather than round, – the world is 3.5 to 5 times the mass of

Jupiter – the closest astronomers have come to

date in finding a system that resembles our own.

– two other confirmed worlds in this system are shown as small dots of light to the left and right of the parent star.

– The innermost gas giant was discovered in 1996 and has a 14.6-day orbit.

– The middle world orbits 55 Cancri in 44.3 days.

• located 137 light years away • in the constellation Orion. • confirmed 0.77 Jupiter mass

planet whips around its star in 14.3 days at an average distance of 0.13 AU.

• There is evidence in the data that a second companion may exist farther out, shown here as a large ringed planet with three satellites.

• The moon close up has icy sheets and ridges similar to those found on Europa and a thin atmosphere

• A second planet has been discovered orbiting Gliese 876, making it one of the most bizarre systems found to date.

• The two planets are eternally locked in sync, with periods of 60 and 30 days.

• Because of this 2-to-1 ratio, the inner planet goes around twice for each orbit of the outer one.

• They gravitationally shepherd one other to maintain this synchrony.

• A lunar landscape is shown at the bottom

• Inner planet has a period a little over three years (1100 days),

• mass about three times that of Jupiter,

• orbital radius about twice the Earth's distance from the Sun.