How Planets Form

jonah

The Heavenly Spheres make music for us,
The Holy Twelve dance with us,
All things join in the dance!
Ye who dance not, know not what we are knowing
~Gustav Holst

By the sweat of your brow you will eat your food
until you return to the ground, since from it you were taken;
for dust you are and to dust you will return.
~Genesis 3:19

Artist's conception of planet formation
Artist’s conception of the formation of planets around the star Beta Pictoris. (Image originally due to NASA. Found on Wikipedia.)

Many months ago, Richard Green posted an article on Google+ that described how life on a toroidal planet would work. The discussion in the comments eventually led to speculation as to whether or not it’s possible for such a toroidal structure to actually form. That discussion is what inspired this post. Today, I’ll tell you about the nebular hypothesis, astrophysicists’ current best theory about how planets form. (Fun historical fact: One of the first people to propose the nebular hypothesis was philosopher Immanuel Kant.)

A torus planet.
A torus-shaped planet. Could such a thing actually form? Sadly, the answer seems to be “no.”  (Source for the background image.)

In The Beginning, Dust

Our solar system probably started as a huge cloud of gas and dust, like a nebula (hence the name “nebular hypothesis”). The particles of dust and wisps of gas gravitationally attracted each other, and other time, the denser clouds of matter collapsed into themselves. As the density of each cloud increased, so did its temperature. Eventually it became so dense and so hot that it became what we call a protostar—a young, vigorous, and confused star.

But not all of the gas and dust had become part of the protostar yet. As the protostar began to age, it began to attract and eat the remainder of the gas. Over time, as the remaining gas fell into the star, it formed a disk, as shown below. Near where the disk touched the star, the compression of its gas superheated it, generating jets of energy pointing out from the star. (For experts: The reason the gas formed a disk is because it had to jet its angular momentum before it could fall in.)

protostar
A newborn protostar feasts on the delicious gas in its accretion disk. Image due to NASA. Found on Wikipedia.

It’s All In The Disk

Let’s talk about the disk some. For reasons that will soon become clear, these disks are called protoplanetary disks. Not everything in the disk orbited around the star at the same speed. As dust and gas particles whizzed past each other, sometimes they collided…and sometimes they stuck to each other, forming bigger particles.

Over millions of years and trillions of sticky collisions, these dust particles grew to several kilometers in diameter, forming small objects that we call planetesimals. At this size, these planetesimals had a gravitational pull strong enough to attract many dust particles, allowing them to grow even faster. Very likely, planetesimals also collided with each other to form even larger objects. Eventually, these bodies were large enough to be called planets.

This description tells us that toroidal planets cannot form. Planetesimals grow in size from collisions with dust and with each other. It seems very unlikely that they would form a shape other than “roughly spherical.”

The End of the Disk

After enough time passed and the protostar had feasted sufficiently, it would begin to emit a blast of light and subatomic particles called the solar wind. This pressure blew away all remaining gas and dust in the disk that hadn’t already coalesced into large objects—leaving behind the collection of planets we know as the solar system.

Evidence

Obviously, we can’t look back in time to see how our own solar system formed. But we can look out into space to try and find solar systems that are in the process of forming. Protostars are easy to capture on film because the jets that they fire have a characteristic shape. We call them Herbig-Haro objects, and they produce some staggeringly beautiful images. See the photos below:

Herbig-Haro objects
Some Herbig-Haro objects. Image courtesy of Wikipedia.

We can also observe the protoplanetary disks that are feeding the protostar. At first, they were hard to spot and we had to use some fancy image enhancement techniques to see them, as in the figure below. The top row shows the unenhanced images and the bottom row shows the enhanced images.

Some protoplanetary disks
Some protoplanetary disks captured by the Hubble Space Telescope. We’ve managed to see what they really are using fancy image enhancement techniques. Image due to NASA. Found on Wikipedia.

However, the Hubble got an upgrade, and we’ve been looking a while. We now have some much better images. Look at the figures below. See those little tadpole-like objects? Those are protoplanetary disks.

more protoplanetary disks
Some prettier images of protoplanetary disks taken by the Hubble telescope. Image courtesy of NASA. Found on Michael Richmond’s course website.

Hubble has now captured more than 150 images of protostars and their jets and disks. A few more for your viewing pleasure:

MOAR disks
Even more images of protoplanetary disks. Image courtesy of NASA. Found on Michael Richmond’s course website.

Sadly, we’re far too distant to see if the disks contain planetesimals.

Issues

I want to emphasize that the nebular hypothesis of planet formation is currently still a hypothesis. It’s the best description we have for how planets formed in our solar system, but it still has some important issues that need to be worked out.

For example, we still don’t know how gas giants like Jupiter form. The planetesimal description relies on the planetesimals being made of rock, so this only describes the formation of terrestrial planets like Earth and Mars. Additionally, the nebular hypothesis predicts that planets should emerge from the disk in very erratic orbits. It’s not clear how they form their current elliptical orbits, which are very regular.

Only time will tell, I suppose, if the nebular hypothesis is correct.

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