Astronomers using the ALMA telescope reported in 2021 the first detection of carbon monoxide (CO) gas in the terrestrial planet region surrounding a young star. Planet formation models predict that in the last stage of planetary assembly, terrestrial planets grow their mass through collisions of planetary embryos, and that these giant impacts should produce observable debris. The ALMA (Atacama Large Millimeter/submillimeter Array) observations of CO strongly suggest the presence of these planet-building collisions, as the gas is an expected by product. The astronomers believe that CO in this system is evidence of a recent planetary collision on the scale of the collision between Earth and another protoplanet that led to the formation of the Moon.