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Ask a Spaceman!

AaS! 178: How Big Can Planets Get?

Ask a Spaceman!

Paul M. Sutter

Astrophysics, Science, Cosmos, Holes, Black, Astronomy, Natural Sciences, Universe, Cosmology, Space, Physics

4.8 • 853 Ratings

🗓️ 21 June 2022

⏱️ 39 minutes

🧾️ Download transcript

Summary

How big can a rocky planet get? Can a rocky planet turn into a gas giant? Why are some planets rocky and others gassy? I discuss these questions and more in today’s Ask a Spaceman!

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Thanks to Cathy Rinella for editing.

SFX CC-0 credit, davidou

Hosted by Paul M. Sutter, astrophysicist and the one and only Agent to the Stars (http://www.pmsutter.com).

Transcript

Click on a timestamp to play from that location

0:00.0

We're talking about the biggest possible planets today, and I decided to divide the episode into two parts.

0:14.0

Easy mode and a hard mode. And we'll do the easy mode first and then obviously do the hard mode second because there's

0:22.3

only two parts in the easy mode comes first anyway the easy mode is asking the literal question

0:27.4

what is the biggest planet and in order to get into this and this will be a recurring theme

0:33.4

throughout the episode when i say biggest or you say biggest, or anyone says biggest,

0:40.0

we need to define what that means because there's a difference between size and mass.

0:48.0

We need to say, or do you mean the most massive planet?

0:51.8

Do you mean the largest planet?

0:53.4

Because there can be large planets

0:55.0

with low density and small planets with high density. Both of them have the equal amount of mass,

1:00.7

so which one is bigger? And we run into this issue, because as we start looking at big planets,

1:08.6

both in terms of size and mass, we start edging into brown dwarf

1:13.5

territory, which itself is hard to define because it exists as a category between stars and

1:20.4

planets. And now we're talking about the lower boundary of that category. We're looking at

1:25.0

what happens when planets become classified as brown dwarfs,

1:28.6

which might just be a category in its own right. It gets complicated real quick because these

1:34.3

boundaries are fuzzy. Usually the bottom edge of a brown dwarf definition is around 20 times

1:41.8

the mass of Jupiter, and the upper edge of the brown dwarf range

1:46.3

is 80 times the mass of Jupiter. That upper end is when hydrogen fusion kicks in, and you've

1:52.2

entered the main sequence, and you're powering yourself for millions or billions or sometimes

1:57.0

even trillions of years. That's a star. You're just definitely a star if you're at least

2:01.6

80 times the mass of Jupiter. At the lower end, the usual lower boundary, and I did a whole

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