Using astrometry to identify candidate exoplanet host stars

HIP 99770 b is the first exoplanet detected through a pioneering approach that combines direct imaging with astrometry, the precise measurements of a star’s motion in the sky. This innovative method holds great promise for enhancing the efficiency of exoplanet searches, bringing us closer to the long-awaited discovery of an Earth-like twin in the vast cosmos.

Traditionally, astronomers have used “blind surveys” to discover exoplanets through imaging, where stars are selected for imaging based on factors like age and distance, without any specific bias. However, these blind surveys have had a low success rate in finding exoplanets. The key to improving detection rates lies in knowing where to look for these elusive celestial bodies.

In their quest to discover exoplanets, an international research team took a novel approach 1. They searched for hints of unknown planets in the data obtained from the European Space Agency’s Gaia mission and its predecessor, Hipparcos. Through their analysis, the team identified a star named HIP 99770, located 133 light-years away in the constellation Cygnus, whose motion indicated the presence of an unseen planet gravitationally pulling on it. This discovery was confirmed through direct imaging observations with the Subaru Telescope, revealing the existence of the exoplanet, HIP 99770 b.

Infrared image of HIP 99770 taken by the Subaru Telescope. The bright main star at the position marked with * is hidden. The dashed ellipse shows the size of Jupiter’s orbit around the Sun for scale. The arrow points to the discovered exoplanet HIP 99770 b. Source: T. Currie/Subaru Telescope, UTSA

The recently discovered exoplanet, HIP 99770 b, is a behemoth, with a mass estimated to be 14-16 times that of Jupiter. Its orbit is approximately 3 times farther from its star compared to Jupiter’s distance from the Sun. Furthermore, this massive planet is scorching hot, being about 10 times hotter than Jupiter. Observations of its atmosphere have revealed the presence of water and carbon monoxide, indicating potential molecular compositions in its gaseous envelope. This intriguing exoplanet, discovered through a combined approach of direct imaging and precise measurements of stellar motion, is providing valuable insights into the diverse and dynamic nature of planets beyond our solar system.

In the near future, astronomers have high hopes of capturing direct images of potentially habitable planets resembling Earth in size and temperature using advanced observatories like the Thirty-Meter Telescope. Unlike the massive and scorching-hot HIP 99770 b, these Earth twins are expected to be smaller and located closer to their host stars, making their detection more challenging. However, with precise measurements of stellar motion, astronomers will have a better understanding of where to search in this cosmic game of planetary hide and seek. The combination of cutting-edge telescopes and refined detection techniques holds promise for unveiling the secrets of potentially habitable worlds beyond our solar system in the coming decade.


  1. Thayne Currie et al. (2023) Direct imaging and astrometric detection of a gas giant planet orbiting an accelerating star. Science doi:10.1126/science.abo6192

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