SCIENCEThe Real Recipe Behind Earth-Like Planets7 min read

The Instrument That Made Precise Measurements Possible

Detecting an exoplanet is one thing. Measuring it accurately enough to determine its internal composition is far harder. The team used the HARPS-North instrument, mounted on the 3.6-metre Telescopio Nazionale Galileo in the Canary Islands. HARPS-North is a high-precision spectrograph designed to detect the subtle wobble a planet induces in its host star as it orbits. By measuring that wobble with exceptional accuracy, scientists can calculate the planet’s mass. Paired with size data from transit observations — where a planet passes in front of its star and dims the light slightly — mass and volume together give density. And density is the key that unlocks composition. Iron and rock are dense. Water and gas are not. The math is straightforward once you have reliable measurements to work with.

Why Kepler-93b Became a Key Test Case

The team focused early attention on Kepler-93b, a planet 1.5 times the diameter of Earth that completes one orbit around its star every 4.7 days. That tight, fast orbit made it an ideal candidate: frequent transits provided abundant light-curve data, and the planet’s gravitational pull on its star was measurable with HARPS-North. What the team found confirmed that Kepler-93b is rocky — not a water world, not a gas planet with a thin rocky shell, but a dense, solid body with a composition consistent with silicate rock and iron. In other words, it matched Earth. That single data point was suggestive, but science does not rest on one example. The researchers needed a larger sample to draw meaningful conclusions.