The observed size of Venus depends on the wavelength of light used to measure it, reports a paper published online in Nature Communications. The research, conducted during the transit of Venus across the sun in 2012, found that measuring the planetary radius of Venus (including the planet and its atmosphere) using extreme ultraviolet and X-ray bands gave values 70-100km larger than the measurements using optical bands.

Venus passes in front of the sun infrequently, with pairs of transits just 8 years apart occurring after long gaps of over 100 years. During the most recent transit of Venus in 2012, Fabio Reale and colleagues were able to use the backdrop of the sun to clearly measure its radius using different, progressively shorter wavelengths, from the optical to X-rays. Their finding that the planet’s radius can actually be up to 100km larger than the standard size has implications for the composition of Venus's upper atmosphere and for the drag that the planetary atmosphere exerts on space probes currently in orbit around Venus. The study indicates the altitude of the dense ion layers of Venus’s upper atmosphere, knowledge that will be important for planning the minimum altitude of any spacecraft fly-bys or entry probes of Venus. The work may also be useful in measuring exoplanets, with multi-wavelength observations marginally feasible today but certainly within the possibilities of future observing facilities on Earth and from space.