Compact Multi-Planet Systems

Happy Thanksgiving! I’m thankful to be living in the age when humanity got to know planets outside our Solar System. I remember being awed when we detected the first one in 1992. I never expected that we’d soon be seeing thousands of them and starting to learn what planets are typically like. That’s actually much more interesting.

We can only detect planets that are large enough and/or close enough to their star, so what we’re seeing is biased. But taking that into account, we still see some real trends—and they’re amazing. There are plenty of solar systems that aren’t at all like ours.

Each row shows a solar system with planets bigger than Earth, and closer to their star than we are to the Sun! Plenty are as big as Neptune: 4 times the radius of the Earth. Some are as big as Saturn: 9 times the radius of Earth. There are even bigger ones, not shown here.

But the really interesting thing is that the planets often act like peas in a pod! They’re often regularly spaced and of uniform size—roughly.

This is something we need to understand. We can try to figure it out by simulating the formation of solar systems.

Why do we need to understand it? Because we live in this universe, and that means some of us can’t resist trying to it! Our realm of concern is spreading beyond the surface of our little planet—though sadly, some still haven’t even learned to care about the whole Earth, and the life on it.

If we look at all planets whose year is less than 1000 of our days, we see more:

There are a bunch of ‘hot Jupiters’ whose year is about 3 days long: that’s the cloud at top here. But there are even more ‘peas in a pod’ solar systems, which have several planets of roughly equal radius, often between the size of Earth and Neptune. A few are shown in different colors here.

These two kinds of solar systems probably require different explanations! For a great talk on this stuff, and especially how hot Jupiters get formed, see Sarah Millholland’s talk “Tidal sculpting of short-period exoplanets”:

After an overview, she focuses on how hot Jupiters form. They’re probably born far from their sun, outside the ‘frost line’:

So what makes some Jupiter-sized planets move in closer to their stars? Maybe interactions with other planets or another star push them into a highly eccentric orbit. Then tides can make them lose energy and spiral closer to their star!

But these tides can work in several different ways—and Millholland goes into a lot of detail about that.

This paper of hers should be good to read for more about the ‘peas in a pod’ phenomenon:

• L. M. Weiss, S. C. Millholland et al, Architectures of compact multi-planet systems: diversity and uniformity, in Protostars and Planets VII, Astronomical Society of the Pacific, 2023.

This is from a conference proceedings, and many of the talks from that conference seem interesting: you can see videos of them here.