We once thought our Solar System was normal, average.
How wrong we were, how varied the planetary systems are out there.
It turns out, the migration of planets played a huge role in the Solar System we see today.
In 1995, astronomers found the first ever planet orbiting another main-sequence star.
The star was 51 Pegasi, and the planet was like nothing astronomers had ever expected
to see.
It has less than the mass of Jupiter, but orbits its star every 4 days.
Even Mercury in our own Solar System needs 88 days to orbit once around the Sun.
But 4 days?
That’s just crazy.
As astronomers continued to search the skies, more and more of these massive planets orbiting
their stars so closely turned up.
A few decades after their first discovery, we now know of dozens of these hot Jupiters.
Not in every system, but it does appear that about 1.2% of stars have hot Jupiters orbiting
around them.
The problem is that hot Jupiters shouldn’t exist.
Old models of planetary formation, based on our Solar System say that you should get rocky
worlds close to the star, gas planets in the middle, and ice worlds farther out.
The blasts of radiation from a newly forming star should have cleared out excess material
from the inner orbits, stopping the formation of worlds.
The only way to explain this situation is if the planets formed farther out, and then
shifted around.
Planetary migration.
Welcome to a two part episode I’m doing with science fiction author John Michael Godier.
If you haven’t yet, you’ve got to check out his channel.
Lots of great space science news and futurism.
In this episode, I’m going to be tackling the science and history of planetary migration.
And in his video, John’s going to be looking forward into the future of planetary migration
in our Solar System, and how we could move the planets around on our own.
I’ll put a link to his episode in the show notes, and in the playlist at the end.
All right, let’s get into it.
Decades ago, astronomers assumed that the planets in the Solar System formed in their
current locations.
The first gas giant is Jupiter with 318 times the mass of the Earth, orbiting the Sun 5.2
astronomical units, or 5.2 times the average distance from the Earth to the Sun.
Then comes Saturn with 95 times the mass of the Earth, orbiting the Sun at 9.6 AU.
Then Uranus with 14.5 Earth masses, orbiting at 19.2 AU and finally Neptune with 17.1 times
the mass of the Earth orbiting at 30.1 AU.
This original theory of Solar System formation couldn’t explain what astronomers were seeing
in reality.
There shouldn’t have been enough material to form Uranus and Neptune so far out from
the Sun.
Furthermore, why is Neptune more massive than Uranus?
That’s pretty strange.
The most commonly held theory about planetary migration is known as the Nice model, named
after an observatory in the French city.
An international group of scientists wrote up a series of papers that explained the configuration
of the Solar System as it looks today.
According to the researchers, billions of years ago, shortly after the Solar System
formed, the giant planets - Jupiter, Saturn, Uranus and Neptune were gathered together
into a much more compact formation.
Jupiter was located about 5.5 astronomical units away from the Sun.
Uranus was the outermost planet, located about 17 AUs from the Sun.
All the giant planets were clustered together, orbiting the Sun in almost perfectly circular
orbits.
Beyond Neptune was a disk of rock and ice extending out to about 35 AUs from the Sun.
Today we see something quite different.
The planets don’t follow circular orbits, they’re more elliptical, orbiting with a
much greater gap between them.
Uranus was rolled over onto its side.
Neptune has a moon that orbits in the opposite direction from every other large Moon in the
Solar System.
Mayhem.
The underlying mechanism came from the transfer of momentum.
Imagine a Solar System with just a single planet going around the Sun, embedded within
a cloud of comets.
As the planet kicks the comets outward, it spirals inward to compensate.
But our Solar System is complex, with multiple giant planets, that would have interacted
with each other in addition to the disk of comets.
Jupiter started at 5.5 AUs and drifted a little inwards.
Uranus and Neptune drifted outward and switched places.
This is why Neptune is more massive than Uranus, even though it’s farther from the Sun.
In fact, it’s believed that Neptune pushed outward to its current location in just 10
million years.
Before the discovery of that first extrasolar planet, we only had a single example of a
planetary system to try and make sense of what could have happened.
Now astronomers have thousands of examples.
The discovery of all these extrasolar planets gave astronomers a vast array of worlds to
run sophisticated computer simulations.
Based on their observations, and simulations, astronomers have developed three kinds of
migration that can occur in a planetary system.
The first, known as Type I is gas driven.
In the early Solar System, the planets orbit within a disk of gas and dust.
As they travel around, gas bunches up into regions of higher density.
The gravity of these regions will cause a planet to gain or lose angular momentum, pushing
it closer or farther from its star.
Larger gas giants like Jupiter clear out these regions pretty quickly, which leads us to
Type II.
This happens after a planet has cleared out the gas in the disk.
Now the mass of the planet itself exerts a torque on the gas around it, pulling it into
the open gap in the disk.
Once again, this causes the planet to lose momentum, making it spiral inward.
This is thought to explain why planets like hot Jupiters can get so close to their parent
stars.
Sometimes this process can get out of control, leading to Type III migration, where other
material enters the gap, causing a runaway migration inward.
In just a few dozen orbits, the planet can be pulled significantly inward.
In a moment, we’re going to talk about the strange migrations that must have happened
out there, but first I’d like to thank:
Rojan Thomas SuperSaiyanFife
Gary Donais
And the rest of our 803 patrons for their generous support.
If you love what we’re doing and want to get in on the action, head over to patreon.com/universetoday.
One of the most fascinating systems out there is Kepler-223, one of the thousands of systems
discovered by NASA’s Kepler space telescope, and it gives astronomers tantalizing clues
about what could have happened here in the Solar System.
We know of four large planets in the Kepler-223 system, all of which orbit very close to their
star.
The closest world takes just 7 days to complete a year, while the most distant one takes 19
days.
The planets themselves are medium-sized worlds, larger than a terrestrial planet like the
Earth, but smaller than a gas giant like Neptune.
We don’t have them in the Solar System, but they seem to be very common across the
Milky Way.
What makes them unusual is that all 4 planets are in resonance, orbiting in lock step with
each other, similar to the orbits of Jupiter’s moons.
For every orbit that Ganymede completes, Europa goes around Jupiter twice, and Io goes around
4 times.
They have settled into their ideal gravitational dance.
Astronomers think that these worlds were the perfect example of planetary migration.
The first planet migrated through the disc of solar material and then got stuck.
A second planet did the same, reached an orbital resonance and got stuck as well, same with
the third, etc.
In our own Solar System, the process was much more violent, with objects smashing into each
other, preventing this kind of resonance.
Even interactions with comets and asteroids would block this resonance, and allow the
giant planets in the Solar System to keep shifting farther and farther outward.
Planetary migration seems to play a pivotal role in the formation of the various types
of solar systems we see across the Milky Way.
Is the migration smooth and stable, leading to close in planets in lock step resonance
like Kepler-223?
Or is it messy and chaotic like what happened here in the Solar System’s early history.
As we discover new planetary systems out there in the Milky Way, we’ll see a wider range
of variations, and better understand how our own Solar System came to be.
I really enjoy thinking about all the exotic worlds out there in the Milky Way.
Did you have any questions or topics you’d like me to investigate about extrasolar planets?
Let me know your thoughts in the comments.
Time for a playlist, obviously the first thing you’re going to want to do is check out
my collaborator, John Michael Godier’s video about the future of planetary migration here
in the Solar System and how we could do it on our own.
Then an older episode of Astronomy Cast where we talk about planetary migration.
Fraser without a beard…
A presentation from Rebekah Dawson to the SETI Instutute.
A talk from Dr. Renu Malhotra about what the search for planet 9 says about planetary migration.
Finally a video from ColdFusion about how astronomers find exoplanets.