All eyes swiveled toward me when the tie-breaker question was asked at last week’s pub quiz: How many times Earth’s mass is that of Uranus? [insert suppressed giggle here]
Think you know? Think carefully. This is pub quiz after all, not Jeopardy—who’s to say the question writers knew the difference between size and mass?
On one hand, gas giant Uranus is definitely bigger than Earth, so logically it must be more massive. But wait, all that gas means it’s probably lighter than our rock-filled home, so maybe it’s less massive? And hey, I know Saturn‘s bigger than Uranus, and I heard it’s got such a low density that it would float in a cosmic-size bathtub!
Here are the facts. Mass is a measure of how much matter an object contains. Period. Mass is the same no matter the size of a thing.
Neutron stars, for example, are relatively small, on average about 12.5 miles (20 kilometers) across. But they pack in the same mass as one and a half sunlike stars.
Uranus is bigger than Earth, coming in at 31,763 miles (51,118 kilometers) across at its equator. That’s almost four times bigger than our planet’s 7,926-mile (12,756-kilometer) diameter.
Meanwhile, the amusingly named gas giant has a mass that’s 14.5 times that of Earth.
By contrast, Uranus’ neighbor Neptune is slightly smaller than it in size—with an equatorial diameter of 30,778 miles (49,532 kilometers)—but it’s 17 times as massive as Earth.
Knowing mass is important, as it determines the values for weight (mass times gravity) and density (mass over volume). [For a lark, enter your Earthly pounds in this planetary weight calculator to see what you’d weigh on some of the other planets and moons in the solar system.]
Combining data about mass and size can tell us whether a planet outside the solar system is more likely to be gassy (bigger like Neptune) or rocky (smaller like Earth).
For the record, the pub quiz writers knew their stuff: The answer to the tie-breaker was 14.5.
It turned out to be a moot point, though, as no teams wound up tied after the final round. So I’ll just leave it a mystery whether our team got it right!