The Sun’s birth cluster and planet-forming disk

This is Chapter 1 in the Solar System’s story. Motivation: you can learn a lot about someone if you know where they came from (or about a planetary system if you know its formation environment).

The curtain lifts on a cloud of dusty, molecular gas floating in the Galaxy. It is about 9 billion years after the Big Bang, or roughly 4.6 billion years in the past. 

[Technical note: we use the ages of the oldest meteorites — which formed about 4.567 billion years ago — to estimate the age of the Sun].

Clumps of gas begin to collapse within the cloud, which is about ten light years across and has a total mass of a few hundred to a few thousand Suns. The centers of clumps reach high enough densities to trigger nuclear fusion to create stars. It looks something like this:

A computer simulation of star formation. Credit: Matthew R. Bate, Ian A. Bonnell, and Volker Bromm

The Sun’s birth cluster

We think the Sun was born in an embedded cluster with about 1000 other stars. But that cluster is long gone, so how could we possibly know how many stars there were?

There are a couple of pieces of evidence that we can use. First, meteorites show the decay products of Aluminum-26 (half-life: 717,000 years). Only very massive stars make Aluminum-26; “normal” stars like the Sun do not. The Solar System’s Aluminum-26 was injected into the Sun’s planet-forming disk by a nearby massive star (perhaps a supernova). Stars massive enough to go supernova are rare: statistically-speaking there is only one massive star for every few hundred run-of-the-mill, normal stars. So the Sun’s birth cluster must have had at least that many.

There is an upper limit on the number of stars in the Sun’s birth cluster. Very massive clusters form up to a hundred thousand baby stars. In a massive cluster, any given star passes pretty close to other stars during its infancy. Really close passages destabilize the orbits of planets far from their stars, like Uranus and Neptune in the Solar System. We also have a disk of icy bodies on nice, very well-behaved orbits called the cold classical Kuiper belt — those would not have survived the close approach of another star either.

In addition, very massive clusters have a bunch of massive stars whose ridiculously-high levels of radiation can evaporate away the disks of stars that pass nearby. Check out this image of an unlucky star’s disk being blasted to oblivion by a nearby O star.

An image of a planet-forming disk being evaporated by the high-energy radiation from a hot massive star, taken with NASA’s Spitzer Space Telescope. Credit: NASA/JPL-Caltech/Z. Balog (Univ. of Ariz./Univ. of Szeged).

The Sun’s birth cluster could have contained between a few hundred and a few thousand stars. This comes from a balance between these competing constraints. Too few star in the Sun’s birth cluster would mean no Aluminum-26. Too many stars and the outer Solar System (and maybe even the planet-forming disk) wouldn’t have survived.

[A side note about Aluminum-26. Even though there was not a huge amount of the stuff, Aluminum-26 produced more heat in the first two million years of Solar System history than all other heat sources combined over the last 4.5 billion years! This is super-important for large bodies: any planetesimal that formed in this early time was completely dried out by radioactive heating.]

The Sun’s birth cluster didn’t last all that long. Clusters in the Galaxy only last for a few million years, or sometimes up to about ten million years. Afterwards, the remaining gas is heated up and spreads out. The gas’ gravity is what holds clusters together, so with the gas gone, stars just drift away in the Galaxy. It’s like the gas cloud holds a bunch of fireflies in its hands and just releases them.

A final twist on the Sun’s birthplace. It is quite likely that the Sun formed quite a bit closer to the center of the Galaxy. The cumulative gravity of spiral arms within the Milky Way causes the orbits of some stars to grow or shrink in a process called “galactic migration” (this is similar to the orbital migration of planets in their stars’ disks that will come up in chapter 3). There is evidence for galactic migration from the compositions of groups of stars, and one-third to one-half of stars in the Sun’s local neighborhood are thought to have originated closer to the galactic center.

Cartoon depiction of galactic migration. The red star has migrated outward whereas the blue star in migrating inward. The Sun today is at about 8 kpc from the Galactic center. Credit: Dana Berry/SkyWorks Digital, Inc./SDSS collaboration.

The Sun’s planet-forming disk

Back to our protagonist — the newly-formed, baby young Sun.

The leftover gas that was moving a little too fast to collapse into a star ends up stranded as a disk in orbit. Our Sun’s planetary disk was spread over an area extending past Neptune’s present-day orbit. It probably contained about 10% of the Sun’s mass, and was about 99% gas (mostly hydrogen with some helium) and 1% solids.

That little speck of solids represents the seeds of all the planets. Actually, that speck wasn’t all that little. Let’s jump into the numbers for a moment. If the disk was 10% of the Sun’s mass and contained 1% solids, that’s one one-thousandth of the Sun’s mass in solids. It doesn’t sound like much, but that’s about one whole Jupiter in rocks — about 300 times the mass of Earth. So, there’s plenty of solids.

Image of the planet-forming disk around HL Tauri, which extends out to about three times Neptune’s orbit around its host star. Credit: ALMA (ESO/NAOJ/NRAO).

We often draw planet-forming disks as perfectly smooth and round, but that’s not what they really look like. New telescopes — especially the ALMA sub-millimeter telescope in Chile — have shown that disks often have spirals or ring-shaped structures. There can be weird shapes in just the dust (where the gas remains smooth) or in both the gas and dust. Ring-like structures seen in images like the one of HL Tauri represent concentrations of dust, and these may well be the birthplaces of planetesimals (as we will see in Chapter 2).

Disks don’t last very long. Within a few million years the gas and dust disperses, never to return. That sets the time window for the formation of any gaseous planets. It’s like those movie trailers: A RACE AGAINST THE CLOCK! If a planet hasn’t captured its gas before the disk is gone, then it never will. (We will come back to this point when we look at gas giant formation).

Disks thin out over their lifetime by slowly draining onto the star. Then, once their densities are pretty low, X-rays and UV light from the central star carve a hole in the disk. The hole gets wider and wider until the disk is fully dispersed. I often describe this process as a donut hole that gets bigger and bigger until the donut is just… gone (gasp!).

Cartoon of the inside-out photo-evaporation of a planet-forming disk. From Ercolano & Pascucci (2017).

Disks can also be evaporated from the outside-in due to the radiation of nearby massive stars (like in the image above). A given star’s disk will disperse on its own if left alone, but the process can be sped up if it passes close to a massive star.

Planet-forming disks disperse on about the same timescale as birth clusters of stars. That means that there can be some interesting interactions between growing planets and other stars. For instance, one possible origins story for Planet Nine (the possible, very wide-orbit ninth planet) is that it was captured from another star in the Sun’s birth cluster. Here is what that might have looked like:

Capture of a possible Planet Nine from another star in a computer simulation (see here). The Sun is initially circled by comets (in red). The smaller encounter star approaches from the top right, orbited by a planet (in black).  The two stars have a close encounter, during which the smaller star is gravitationally deflected. The Sun captures the planet (black symbol) from the second star, which itself captures a number of comets from the Sun. The background color shows the strength of the local gravity field. Credit: Alex Mustill.


The TL;DR version of this post: the Sun formed in a cluster of about 1000 stars. The Sun’s gas-dominated disk disappeared in a few million years, about the same timescale as the cluster’s dispersal.

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