It marks the first X-ray confirmation of such events in the 20 years since exoplanets were first discovered, the space agency reported Monday.
"Thousands of planet candidates have been seen to transit in only optical light," said Katja Poppenhaeger of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "Finally being able to study one in X-rays is important because it reveals new information about the properties of an exoplanet."
An advantageous alignment of a planet and its parent star in the system HD 189733, 63 light-years from Earth, enabled NASA's Chandra X-ray Observatory and the European Space Agency's XMM Newton Observatory to observe a dip in X-ray intensity as the planet transited the star, NASA said.
The planet, known as HD 189733b, is a hot Jupiter, meaning it is similar in size to Jupiter in our solar system but in very close orbit around its parent star.
Astronomers previously used NASA's Kepler space telescope to study it at optical wavelengths and NASA's Hubble Space Telescope to confirm it is blue in color as a result of the preferential scattering of blue light by silicate particles in its atmosphere.