# What does it mean for a planet to be just 300 kms across?

The recently discovered Planet Goblin is just 300 kms across. While I perfectly understand what it literally means, I'm having trouble wrapping this in my head. Does it mean I can imagine the planet as (approximately) a sphere for 300 kms diameter? That gives a radius of 150 kms, and thus a circumference of 942 kms.

A circumference of 942 kms means that the longest distance between any two points on the surface of the planet is just 471 kms -- That's just over 1.5 times the distance between London to Paris or 100 kms more than the distance between New York and Washington DC or about half the distance between St. Petersburg and Moscow.

Is the planet that small, really? Or am I missing something?

• It's referred to as a dwarf planet. At that size it might be spherical (if it's made of something relatively soft, like ice) or it might be more irregular if it's made of something relatively hard like rock. I imagine all we know is how much light it reflects in total. So it could also be bigger but unexpectedly dark, or very shiny but smaller. Commented Oct 2, 2018 at 15:35
• Size is size. That's it. (Still rather larger than the home planet of Le Petit Prince. Commented Oct 2, 2018 at 19:27
• The diameter is based on the absolute magnitude and an assumed albedo (amount of light it reflects). The absolute magnitude of 5.3 is below (fainter) than the 4.0 normally assumed for dwarf planets. So it's "just" a Trans-Neptunian Object with a cool (and somewhat uncertain) orbit Commented Oct 3, 2018 at 5:56