Nearby star hosts a half-dozen synchronized sub-Neptunes

The most frequently observed variety of extrasolar planet doesn’t have a representative in our solar system. Sub-Neptunes , with diameters that fall between that of Earth and Neptune, orbit the majority of Sun-like stars in the galaxy, according to statistical studies of the several thousand exoplanets discovered to date. With no example around our sun to examine, planetary scientists hope to study sub-Neptune exoplanets to understand how they form and evolve. Now Rafael Luque of the University of Chicago and colleagues have found that a nearby star is surrounded by six tightly orbiting planets—all of them sub-Neptunes.

Initial evidence for the multiplanet system came from NASA’s Transiting Exoplanet Survey Satellite (TESS), which for the past five years has stared at bright stars across the sky in its hunt for transits—shadows cast by planets crossing the face of their host stars. In 2020 the telescope spotted two candidate planets around HD 110067, which is located about 100 light-years away. The next time it gazed at the star, in 2022, it recorded transits that were potentially attributable to two additional planets. Analyses of the TESS data combined with observations by the European Space Agency’s Characterising Exoplanets Satellite enabled the confirmation of three planets, but some of the transits remained unexplained.

The key to uncovering more of HD 110067’s planetary system was recognizing the pattern of the orbital periods of the star’s three known worlds. The third planet from the star, HD 110067 d, takes about 1.5 times as long to complete an orbit as HD 110067 c, which takes 1.5 times as long as the innermost planet, HD 110067 b. Each pair is in a 3:2 orbital resonance, a common configuration shared by bodies such as Pluto and Neptune. After simulating various configurations of additional planets, the researchers proposed that the outstanding transits resulted from the existence of three more planets that are in a 4:3 resonance with each other.

By analyzing the dips in stellar flux during each planet’s transits, the researchers determined that the diameters of the six planets range from 1.9 to 2.8 times that of Earth—all prime examples of the sub-Neptune worlds that our solar system lacks. Ground-based measurements of the planets’ gravitational influences on their host star helped narrow down some of the planetary masses. The observations suggest that HD 110067 b, d, and f are low-density worlds resembling our solar system’s outer planets. All the planets are located too close to their star to host liquid water on their surfaces.

With six sub-Neptune planets of different sizes and temperatures in a shared environment, the HD 110067 system could be a valuable test bed for planetary formation and evolution theories. The planets’ resonant orbits suggest that the system hasn’t changed much in its 4-billion-year history. Astronomers suspect that nearly all planetary systems begin in resonance, but in most cases catastrophic events upend that delicate synergy. And between the star’s proximity and its considerable mass of about 80% that of the Sun, HD 110067 has the greatest apparent brightness of the stars known to host at least four planets. That means astronomers can collect more of the precious photons they need to further investigate the star’s planetary companions. (R. Luque et al., Nature 623, 932, 2023 .)