exoplanets | Ask Dr. Universe | Washington State University

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How do scientists know the mass and size of planets? – Lucy, 10, Illinois

January 8, 2026 melissamayer

Dear Lucy,

When I go to the veterinarian, they measure how big I am. They use a scale to check my weight. They find my length and height with a measuring tape.

But nobody can put a planet on a scale. Or wrap a measuring tape around it.

I asked my friend Katie Cooper about it. She’s a geologist at Washington State University. She studies our planet and other planets, too.

She told me we send satellites to orbit our solar system. When a satellite swoops around a planet, it snaps pics and takes measurements. Those images and measurements help scientists estimate the … » More …

Besides telescopes and spaceships, how do we really know that there are other planets? – Ia, 12, Montana

September 29, 2023 melissamayer

Dear Ia,

I looked through a high-power telescope for the first time in college. I couldn’t believe how many stars I saw. It’s hard to imagine all the planets orbiting all those stars.

I talked about how we know those planets are out there with my friend Jose Vazquez. He’s an astronomer at Washington State University.

He told me that scientists look for planets outside our solar system using a number of instruments—like a photometer. That's a tool that attaches to a telescope and measures light.