Astronomers Weigh Two Small Planets Around a Sun-Like Star and Spot a Hint of a Third

Astronomers have placed new constraints on two small planets orbiting K2-223, a star much like the Sun, and one of them is smaller than Earth while completing an orbit in about half a day. Dawid Jankowski and colleagues describe the results in a paper posted to the arXiv preprint server on Sept. 1.

Both planets were already known from the dips in starlight they cause when they cross in front of the star, a method that reveals their sizes. Measuring or constraining their masses alongside those sizes provides clues to their compositions. The team also reports a possible third planet, much larger and much farther out. Two small worlds close in with a giant beyond them would make K2-223 a useful test case for ideas about how planetary systems form and evolve.

The mass constraints come from the HARPS-N and ESPRESSO spectrographs, which detect the slight back-and-forth motion a planet's gravity induces in its star. For the inner planet, K2-223 b, the signal is too faint to produce a direct mass measurement. The paper reports a radius of 0.79 times Earth's and only an upper mass limit: less than 2.8 Earth masses at three sigma. The authors state that this makes it currently the smallest known ultra-short-period planet transiting a Sun-like star.

The outer of the two, K2-223 c, has a measured mass of 4.2 ± 1.3 Earth masses. Its radius is 1.41 ± 0.15 times Earth's, its density is 8.3 ± 3.7 grams per cubic centimeter and its orbit lasts about 4.5 days.

The candidate third planet emerged from almost six and a half years of monitoring with HARPS-N. The authors call the detection tentative, and the radial-velocity method provides only a minimum mass: 1.29 times the mass of Jupiter, on an orbit of 4.53 years.