In the last few decades, more than 4000 planets have been discovered orbiting other stars. These extrasolar planets, or exoplanets, span a vast range of properties, from "hot Jupiters"—gas giants that are closer to their stars than Mercury is to our Sun—to tightly-packed systems of multiple "super-earths" orbiting faint red stars. In the outskirts of distant stellar systems, where only the direct imaging technique is sensitive to planets, scientists at KIPAC have captured images and spectra of young super-Jupiters to learn about their formation and evolution.

At KIPAC, astronomers use a revolutionary instrument called the Gemini Planet Imager (GPI) to capture light from planets orbiting distant stars. Images and spectra collected with GPI allow KIPAC astronomers to study a distant world’s orbit, atmospheric composition, temperature, age, and other characteristics. By combining these observations with advanced statistical analyses, we study how common these distant giant planets are, and try to determine how they formed. 

Direct imaging is uniquely suited to discovering giant planets at wide separations, at or beyond the location where the Solar system giant planets are located. An active area of research is measuring how frequently giant planets form at these larger orbital radii, and whether they form in the same way as close-in giant planets. When we look at other stars, do we see one population of giant planets at a large range of orbital periods, or are there multiple distributions, tracing different formation mechanisms or migration histories? Since the occurrence rate of these wide-separation giant planets is less than 10%, we need a direct imaging survey targeting hundreds of stars with high sensitivity to planets to answer this question.

The GPI Exoplanet Survey (GPIES) was designed to answer big questions about the formation of wide giant planet systems orbiting nearby stars. The survey targeted 600 young nearby stars,  and detected six planets, including the first discovery of the planet 51 Eridani b. Detailed spectroscopic measurements of the atmospheres of these planets help determine whether they formed by accreting solid icy or rocky planetary building blocks (as we think the planets in our solar system did) or by the collapse of clouds of gas (as we think stars form). 

By combining our detections with a careful analysis of the overall sensitivity of the survey, we have determined that only approximately 7% of stars host planets of this type. Interestingly, massive A-type stars are more than twice as likely to host this type of planet (about 11%) than lower-mass sun-like stars (<5%). This suggests that something qualitatively different happens when planets form around stars of different masses: either there are different formation mechanisms at play, or else the scales of the giant planet formation zone depend sensitively on the mass of the star. 

In its current state, GPI can only detect very massive, young planets—the equivalent of our Jupiter over four billion years in the past—which shine brightly in the infrared with the heat of their recent formation. But KIPAC scientists are working on technological upgrades to improve the sensitivity of GPI and similar instruments. Ultimately, the same technology will be applied to study Earth-like planets, allowing us to probe their atmospheres and hunt for chemical compositions that could indicate life.