Ars Technica: The Kepler telescope searches for distant planets by looking for minute variations in their host star’s brightness, which are caused by the planets passing in front of it. The telescope’s newest discovery is a system of three planets all with orbital radii less than Mercury’s. They have been designated Kepler-37b, c, and d, based on their distance from their parent star. Kepler-37d was determined to have a diameter twice that of Earth and an orbital radius about one-half that of Mercury. The diameter of Kepler-37c is 75% of Earth’s, and its orbit is about one-third of Mercury’s. The signal revealing Kepler-37b is so weak that it took a statistical algorithm to tease it out. The diameter of the planet is just 30% of Earth’s, and it orbits in 13 days with an orbital radius one-quarter of Mercury’s. The planet’s size and proximity to its sun suggest that it is rocky and even hotter than Mercury, and that it may be tidally locked. The unusual nature of the system adds to the ever-growing understanding of the wide diversity of planetary bodies and systems.