Earth-like planet masses remain hard to measure around Sun-like stars

A test of methods for weighing planets found that measuring the masses of habitable-zone planets around Sun-like stars to better than 10% remains difficult. With data like those tested, the limit is typically around 8 to 10 Earth masses. The results suggest that smaller planets around orange dwarfs may be measurable, and offer a threshold for separating strong signals from candidates.

Star system: Tau Ceti | Distance from the Sun: 11.9 light years

Measuring the masses of small planets around Sun-like stars remains challenging. In a test of methods for analysing star movements, teams typically could not measure the signals of habitable-zone planets to better than 10% precision unless the planets had masses of about 8 to 10 Earth masses. A habitable zone is the range of distances where a planet could have conditions suitable for liquid water on its surface.

The test used observations of the Sun treated as if it were a distant star, collected with the HARPS-N instrument. Researchers created data for three systems containing multiple planets, with realistic observation timing and about 1,000 measurements in total. Eight teams independently tried to recover the planets' radial-velocity signals. Radial velocity tracks the slight movement of a star caused by an orbiting planet. The size of that signal, called its semi-amplitude, helps reveal the planet's mass.

The results depend on the instrument stability, observing plan and analysis methods represented in the test. For orange dwarfs, which are cooler and less massive than the Sun, the same level of precision may be possible for planets weighing about 2 to 5 Earth masses. The teams also used the results and the rate of false positives to set an empirical detection threshold. This threshold can help distinguish robust planet detections from signals that remain candidates. The data are publicly available as a benchmark for comparing methods used to analyse radial-velocity measurements.

Key findings
For habitable-zone planets around Sun-like stars, better than 10% precision is typically challenging below about 8 to 10 Earth masses.
For orange dwarfs, planets around 2 to 5 Earth masses may be characterised at similar precision.
Eight teams tested methods using about 1,000 observations across three simulated multi-planet systems.
An empirical radial-velocity detection threshold can help separate robust detections from candidates.
The challenge data are publicly available for comparing analysis methods.

Scientific study

Measuring masses of transiting Earth-like planets: first insights from the PLATO RV data challenge (external link)