Can planets share orbits? A new observation implies they can!

July 24, 2023   Issue #594

Subscribers lead me 60° around in my orbit

Shameless Self-Promotion

Where I’ll be doing things you can watch and listen to or read about

I was interviewed by my colleague (fellow astronomer and science communicator) Dean Regas of the Cincinnati  Observatory about astronomy and space conspiracies! UFOs, the Moon Hoax, the Sun exploding… which do you think I secretly wish were true?

Listen here and find out.

We also talk about life in space, including other Earths, icy moons, and more. It was a lot of fun!

This issue of the Bad Astronomy Newsletter is free to the public, so if you like it and think someone else will too, go ahead and share it! Just click the button below and add in some friends. Easy peasy chicken dinner.

Astronomy News

It’s a big Universe. Here’s a thing about it.

A common idea in scifi stories is two planets sharing the same orbit, usually with civilizations on each that are at war or some other trope that can be used as allegory for humans on Earth.

But can that happen? Not the warring aliens — well, maybe, who knows? — but the two planets sharing a single orbit around a star? Theory says it’s possible, even if we don’t see such a thing in our own solar system.

New research using one of my favorite observatories, ALMA, may provide the first solid evidence that this situation exists in nature. And what they found does have some similarities to what we see in our own solar system, though not to this extreme.

They observed the nearby star PDS 70, which is about 370 light-years from Earth [link to research paper]. It’s extremely young, just about 5 million years old, and is still surrounded by a flat ring of gas and dust from which it formed. This is called the protoplanetary disk, meaning planets are forming from it. And in fact two planets have been found; PDS 70b and c, both of which are gas giants like Jupiter, though about three times more massive. Both were detected using direct imaging, meaning they were actually seen in images taken of the system.

The gravity of the planets affects the disk; 70c, the outer one, has presumably carved the inner edge of the ring, pulling in material and truncating the inner part of the disk.

For this new work, astronomers looked at archival ALMA data taken of PDS 70 taken in 2019, and reprocessed it using new techniques to help clean it up. When they do, they found a small blob of light near PDS 70b. It’s fainter, but still bright enough that we can be reasonably sure it’s not just random noise but an actual, physical object, likely a cloud of dusty debris.

OK, that’s neat. But where this gets so very interesting is where this blob actually is. We see this disk tipped to us by an angle of about 50°, so the circular ring looks like an ellipse. If you correct for that, you find that not only is the blob at the same distance from the star as PDS 70b, but it’s also very close to being 60° around the orbit from the planet.

And oh my, that’s interesting indeed.

In the 1800s, mathematician Joseph-Louis Lagrange published “Essay on the three-body problem” (which you may have heard of under other circumstances). In it, he shows mathematically what happens when you have three objects orbiting each other. In the case where one is very massive, like a star, and the second less massive, like a planet, there exist five points in space where the gravitational and centrifugal forces balance out in such a way that an object placed in one of these points can remain there. Three of them are unstable; a slight push on an object there will send it flying off.

But two of them, called the L4 and L5 points (L for Lagrange), are stable. If you put an object there it will tend to stay there. These points are, respectively, 60° ahead and behind the secondary object; in this case, a planet.

Jupiter is the largest and most massive planet in the solar system, and therefore the most gravity. There are a lot of asteroids in its L4 and L5 points, perhaps over a million, though most are tiny (the wonderful space mission Lucy is on its way there to look at some of the bigger ones, in fact). They likely wandered in from other parts of the solar system, but due to the gravitational stability of those points they stayed there, sharing Jupiter’s orbit. As I write in that article linked above, for historical reasons these are called the Trojan points, since the first few asteroids were named after heroes of the Trojan War, and traditionally they have been since (though there are some fun exceptions). Earth has two confirmed Trojan asteroids, too.

OK, great. But now we can extrapolate a little from asteroids wandering into a planet’s Trojan points to, perhaps a whole planet being there.

When planets form, they collect from particles of dust that stick together to form grains, then pebbles, then boulders, then bigger objects called planetesimals, which then slam together to create proper planets. Once the gravity of this planet is large enough it can actively draw in material and grow that way. I’ve skipped a few steps there but that’s the general idea.

Once the protoplanet is big enough, its gravity can set up stable Trojan points in its orbit. Material won’t just wander in, but can actually actively collect there, like rain water collecting in a dip in the road. If enough material collects there, it might be able to form a planet on its own.

So the theory goes. But these ALMA observations provide some compelling evidence that this theory has some game.

The amount of material seen in the blob sharing PDS 70b’s orbit can be calculated by the brightness in the images, and it’s actually pretty low, about twice the mass of the Moon, roughly 2.5% of the Earth’s mass. Not much. But it’s possible there’s a planet inside all that junk with more mass. Or it could be that two smaller planetesimals collided there, creating the cloud, in which case there may not be a planet there at all.

It’s also possible that the cloud really is just noise in the data, or a random collection of material that happens to be where we’d see PDS 70b’s L5 point. The way to tell is wait a few years and see if the material is moving along a path consistent with PDS 70b’s orbit. If it is, then that’s really good evidence it’s actually L5 material. The astronomers note the earliest date they can do that is early 2026. The longer they wait, though, the more it’ll move and the easier it’ll be to see what’s what.

I’ll add that something to this has been seen twice before in protoplanetary disks, but in neither case is there an actual confirmed planet found in the right location; only the Trojan material is seen, if that’s indeed what it is. In this case, though, we know PDS 70b exists, so this system is a step ahead of the other two.

I really hope this turns out to be real. If this shows that two planets can a share an orbit, then this — once again — shows us that nature is pretty imaginative and loves to be creative with the tools it has.

Et alia

You can email me at [email protected] (though replies can take a while), and all my social media outlets are gathered together at about.me. Also, if you don’t already, please subscribe to this newsletter! And feel free to tell a friend or nine, too. Thanks!