Question 1

How big can a planet get?

Accepted Answer

Radius stops growing long before mass does. Add material to a gas giant and gravity compresses what is already there, so planets from about one to eighty Jupiter masses are all roughly one Jupiter radius — above that, fusion starts and the object is a brown dwarf rather than a planet.

Question 2

Why do some look larger than they should?

Accepted Answer

Because they are hot. A giant planet orbiting close to its star is heated until its atmosphere puffs up, and several of the largest here are inflated hot Jupiters with densities lower than cork.

Question 3

What does the size column actually measure?

Accepted Answer

Not always the same thing, and the site says which. A round body has a mean radius; an oblate planet may have an equatorial one; an irregular asteroid has an effective radius, meaning the radius of a sphere that would show the same disc. Each object's page names the kind it holds.

Question 4

How firm is the order?

Accepted Answer

Firm at the top and soft further down. The largest bodies have been resolved or visited and their sizes are known to a per cent or two; the smaller ones are often sized from brightness plus an assumed reflectivity, which can be wrong by a factor of two in either direction.

Question 5

Why are some exoplanets missing?

Accepted Answer

Because no size is recorded for them here. Measuring one needs a resolved image, an occultation, a transit or a radar echo, and most catalogued bodies have had none of those.

Question 6

How is a planet's size measured?

Accepted Answer

From the depth of its transit: the fraction of the star's light it blocks gives the ratio of the two radii, so the planet's size is only as good as the star's. A revision to the star's radius moves every planet around it.

Question 7

Where is the line between a planet and a star?

Accepted Answer

Around thirteen Jupiter masses, where deuterium begins to fuse, and around eighty, where ordinary hydrogen fusion starts and the object is a star. The middle band is the brown dwarfs, and the boundary is a convention rather than a sharp physical change.

Question 8

Why are so few small planets on lists like this?

Accepted Answer

Because this one ranks by size, and small planets are also the hardest to detect. Corrected for that bias, planets between the size of Earth and Neptune appear to be the commonest kind in the galaxy — and this Solar System has no example of one.

The largest exoplanets

COMMON QUESTIONS