Which exoplanet was the first to have an X-ray eclipse observed?

HD 189733 b was the first exoplanet to have an X-ray eclipse observed. In 2013, NASA’s Chandra X-ray Observatory and the European Space Agency’s XMM-Newton detected a dip in the X-rays from its host star as the planet passed in front of it.

HD 189733 b is a “hot Jupiter,” a gas giant orbiting extremely close to its star, HD 189733 A. It completes one orbit in about 2.2 days and lies roughly 63 light-years from Earth. Its close orbit and large apparent transit made it an especially useful target for repeated observations.

The X-ray eclipse was deeper than the corresponding visible-light transit—about three times deeper. That suggested the planet possesses an extended upper atmosphere that blocks X-rays beyond the region measured in ordinary optical observations. Researchers estimated that intense stellar radiation is gradually stripping this atmosphere away.

This result is easy to confuse with other famous exoplanet firsts. HD 209458 b was among the first exoplanets studied spectroscopically, while HD 189733 b became notable for its blue color, infrared observations, and the first observed X-ray transit.