New Developments in the Formation of the Solar System

Steve DeschArizona State University

Outline

What was the solar nebula like? How massive was it, how large was it, how did it evolve, what about its surroundings?

First, the old story: large, low-mass disks in a low-mass star-forming region like Taurus.

New evidence for a smaller but more massive disk, from planets and meteorites.

New evidence for a birth in a stellar cluster, much more like the Orion Nebula.

The “Old” StoryNot long ago, astronomers considered the Taurus Mole-cular Cloud to be a typical star-forming environment...

The “Old” Story

...and the solar systems that formed there to be typical.

HST WFPC2/NICMOS

100 - 1000 AU

Accretion disks in Taurus

But Taurus is not typical. It’s just close.

Taurus = 130 pc

Orion = 450 pc

Sco-Cen= 450pc

The “Old” Story

Only 10-30% of Sun-like stars form in regions like Taurus (Lada & Lada 2003).

Most form in regions like the Orion Nebula that contain at least one really massive star.

1Ori C: imminent supernova

disks

~0.2pc

Disks forming in these environments are much smaller than in Taurus-like regions (< 50 AU), shaped by photoevaporation by ultraviolet radiation, by stellar winds, by stellar close encounters, and by supernovae.

Are these the type of environments in which our Solar System formed?

Meteorites are one way we can find out!

MATRIX GRAINS CHONDRULES CAIs

Cross section of Carraweena (L3.9) ordinary chondrite

1 mm

Chondrules are millimeter-sized, ferromagnesian melt droplets.

Their textures are reproduced only if they temperatures > 1575 C, for minutes, then cooled over a matter of hours.

Chondrules probably formed when they were hit by shock waves in the gas of the solar nebula.

Melting by nebular shocks is consistent with all known properties of chondrules, but only if the gas density is ~ 1 x 10-9 g cm-3

Temperature and cooling rates of a chondrule passing through a 7 km/s shock

Desch & Connolly (2002)

About 5% of chondrules are compound chondrules, stuck together while molten (Ciesla et al. 2004).

Assuming relative velocities < 100 m/s (!), there must have been > 10 chondrules per cubic meter (Gooding & Keil 1981).

Chondrules lost K, Mg, Fe and Si while melted, but no isotopic fraction-ation is measured.

Implies rock vapor stayed in the vicinity of chondrules: need > 10 chondrules per cubic meter, in clouds > 1000 km across (Cuzzi & Alexander 2006)

Chondrule densities nc ~ 10 m-3 are high!

nc = (c / gas) gas

4/3 s ac3

= C (0.005) (10-9 g cm-3) (gas / 10-9 g cm-3)

4/3 (2.5 g cm-3) (300 m)3

nc = 0.015 C (gas / 1 x 10-9 g cm-3) m-3

Chondrules probably were concentrated,

but probably not by factors C > 200.

The upshot: chondrules had to form in a region with gas density ~ 1 x 10-9 g cm-3, greater than had been thought possible at 2 or 3 AU.

Gas surface density is estimated using “Minimum Mass Solar Nebula” (Weidenschilling 1977)

If a disk thickness is estimated, the gas density is derived.

Minimum Mass Solar Nebula

M

M

J

EV

S

U

N

This model yields densities that are too low to melt chondrules by shocks, by a factor of about 5 (i.e., gas / 1 x 10-9 g cm-3 at 2-3 AU).

A bigger problem: Jupiter, Saturn, Uranus & Neptune can’t accrete H2 & He gas until their rocky cores reach about 10 Earth masses in size.

No models of planet growth predict such a large planet can form in < 10 Myr at 5.2 AU, unless the density of the solar nebula is at least 5 x the minimum mass solar nebula.

Even then, growth of Uranus and Neptune at 19 AU and 30 AU impossible in < 5 Gyr.

Problems with the Minimum Mass Solar Nebula

According to the `Nice Model’, the Giant Planets did not form where we find them today!

Here’s where they started:

Jupiter = 5.45 AU, 12.7 yr (5.2 AU, 11.9 yr)

Saturn = 8.18 AU, 23.4 yr (9.6 AU, 29.5 yr)

Neptune = 11.5 AU, 40 yr (30.1 AU, 165 yr)

Uranus = 14.2 AU, 54 yr (19.2 AU, 84 yr)

+ 35 Earth Mass Disk of ‘Planetesimals’ = 16 - 30 AU

Enter the Solution: the Nice Model

Uranus scattered planetesimals inward, which gave `gravity assists’ to the planets

The Nice Model

Jupiter moved inward, Saturn moved out, until they reached a 2:1 resonance (after about 700 Myr)... Solar System went chaotic, driving Neptune and Uranus out.

The Nice Model

A close encounter between Uranus and Neptune led to them switching places!

Continued migration rapidly depletes planetesimal disk, sending some in toward Earth.

The Nice Model

Many planetesimals are lost, some scattered into the Oort cloud, and many are scattered into the modern-day Kuiper Belt

Neptune stops migrating when the number of planetesimals it can scatter gets too low.

The Nice Model

This happens 10-20 Myr after Jupiter & Saturn went chaotic (700 Myr after Solar System birth). Solar System has been stable ever since (for 3.9 Gyr).

Pluto & Kuiper Belt

Planets start on circular, coplanar orbits, but end up on slightly eccentric, inclined orbits.

Gomes et al. (2005)

Nice Model explains why the giant planets have the orbits they do, and why the Late Heavy Bombardment of the inner solar system occurred (and the structure and size of the Kuiper Belt, and Jupiter’s Trojan asteroids, etc., etc.).

If true, the planets formed closer to the Sun, which speeds up their formation, but still not < 10 Myr.

However, if the planets formed closer together, the Minimum Mass Solar Nebula must be wrong!

The planets were spread out from 5-15 AU, not 5-30 AU. One quarter the area = 4 x denser!!

Desch (2007)

Amazing conformance of diverse data to a single trend!

But density falls off very steeply with radius, too steeply to be an accretion disk...

Not possible to have a steady-state disk with mass moving in and have that profile...

But it’s a prediction (Desch 2007) if the disk is being photoevaporated and gas is steadily moving out while the planets form!

Photoevaporating disks in the Orion Nebula (HST)

The steady-state decretion disk solution of Desch (2007) is very favorable for planet growth... all four giant planets form in < 10 Myr!

Outward flow of mass also explains how CAIs ended up in Comet 81P/Wild 2, sampled by Stardust (Zolensky et al. 2006)

Speaking of CAIs.....

In the first few Myr after its birth, the Solar System contained live radioactive elements with half-lives ~ 1 Myr.

26Mg 26Mg 26Al 27Al

24Mg 24Mg 27Al 24Mg

27Al / 24Mg

26Mg

24Mg

= + x

41Ca 0.1 Myr 41Ca/40Ca = 1.4 x 10-8

36Cl 0.3 Myr 36Cl/35Cl = 3 x 10-6

26Al 0.7 Myr 26Al/27Al = 5 x 10-5

60Fe 1.5 Myr 60Fe/56Fe = 5 x 10-7

10Be 1.5 Myr 10Be/9Be = 9 x 10-4

53Mn 3.7 Myr 53Mn/55Mn = 1.4 x 10-5

107Pd 6.5 Myr 107Pd/108Pd = 2 x 10-5

182Hf 9 Myr 182Hf/180Hf = 2 x 10-4

129I 15.7 Myr 129I/127I = 1 x 10-4

SLR Half-life Initial Abundance McKeegan & Davis (2003)

probably from a nearby supernova

unique source

probably part of the inter-stellar cloud gas from which Solar System formed.

definitely a supernova!

Cosmic Ray Origin of 10Be

10Be Galactic Cosmic Rays must have been trapped in our molecular cloud core. Trapped GCRs match the meteoritic 10Be/9Be. Uncertainties are factors of 2-3 total. At least a third, and probably all, of the 10Be in the early Solar System attributable to cosmic rays!

meteoritic 10Be/9Be

Desch et al. (2004)

Schleuning (1998)

spallation

total

trapped

Supernova Origin for 26Al, 60Fe, etc.

1Ori C: imminent supernova

disks

~0.2pc

~ 0.4 pc

Pismis 24-1 (O3 If*), Pismis 24-17 (O3 IIIf*) and Wolf-Rayet stars (Massey et al. 2001)

These stars will supernova in < 1 Myr

G353.2+0.9 H II region in NGC 6357 (Healy et al. 2004; Hester & Desch 2005)

Protoplanetary disks ~ 0.3 pc from a supernova (1051 erg) are not destroyed! (Chevalier 2000; Ouellette et al. 2005; Ouellette et al. 2007)

Ouellette et al. (2007)

What will happen to those disks?

Gas is not mixed in well, but dust grains are! (Ouellette et al. 2007)

Iron likely in form of dust grains: gas-phase Fe disappeared from SN 1987A ejecta at same time (2 years post-explosion) that 10-3 M of dust formed (Colgan et al 1994)

Mass of 60Fe ejected by 25 M supernova ~ 8 x 10-6 M (Woosley & Weaver 1995)

Fraction intercepted by 30 AU radius disk at 0.3 pc away ~ (30 AU)2 / 4 (0.3 pc)2 ~ 6 x 10-8

Mixed with 0.01 M of solar composition material, 60Fe / 56Fe ~ 1 x 10-6

One supernova can also inject the other short-lived radionuclides with observed abundances (Ellinger et al. 2007, in preparation)

Injection of radioactive grains directly into protoplanetary disk supplies just enough 60Fe!

Short-Lived Radionuclides

Conclusions

Our Solar System grew up in a tough neighborhood!

Models of the formation of chondrules in meteorites tell us the disk was very dense and did not spread out much.

New models of the “minimum mass solar nebula” show the nebula really was very dense, but this structure is only understood if the disk was photoevaporating because of a nearby massive star.

Meteorites show the Solar System contained live 60Fe, almost certainly ejected by a nearby massive star that went supernova.

The Sun formed in a region much more like the Orion Nebula than the Taurus molecular cloud!