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Venus has a core that is approximately 7000 kilometres in diameter – about the same size as Earth’s – according to the first observation-based estimate.

Studying Venus is notoriously difficult owing to its thick atmosphere, which hides the surface. As such, radar and other specialist observation techniques are required to probe beneath its abundant clouds.

Jean-Luc Margot at the University of California, Los Angeles, and his colleagues examined Venus from 2006 to 2020, using the Goldstone Solar System Radar in California to hit the planet with radio waves. They then used both this and the Green Bank Telescope some 3000 kilometres away in West Virginia to track the echoes of the waves as they bounced back to Earth, a technique called radar speckle tracking.

This allowed them to measure very small changes in Venus’s spin and movement. They found the planet’s day, roughly equivalent to 243 Earth days, fluctuated by up to 21 minutes over the 15 years of observation. They also found that the axis of Venus wobbled very slightly in a pattern that their calculations suggest would repeat every 29,000 years.

While the latter is the result of the sun’s pull, the main driving factor behind the former is the planet’s thick atmosphere, which pushes and pulls the surface. But they suspect another factor is Venus’s core, and have used their data to calculate how large a core would be needed to explain the fluctuations.

“We have a rough estimate of about 3500 kilometres [for the core’s radius],” says Margot. While the team was unable to deduce if the core was liquid or solid, previous theoretical work suggests it is mostly made of iron and nickel like our own. However, it isn’t known “if Venus has an inner solid core and an outer liquid core, like Earth, or if it’s all solid or all liquid”, says Margot.

While the estimate is in line with previous models of the core size, having an actual measurable value will allow more accurate studies of Venus in future. Knowing the core’s size and density could be useful in understanding the history of the planet, for example.

“Pretty much everything about a planet’s evolution is dictated by the size of the core,” says Margot. “It’s really difficult to understand anything about a planet unless we have a good picture of its interior structure.”