Hubble and New Horizons offer dual perspectives on Uranus to inform exoplanet research

NASA’s Hubble Space Telescope and the New Horizons spacecraft recently set their sights on Uranus, allowing scientists to make a direct comparison of the planet from two very different perspectives. The results inform future projects to study similar types of planets around other stars.

Astronomers have used Uranus as a proxy for similar planets beyond our solar system, called exoplanets, comparing the high-resolution images from Hubble to the more distant view from New Horizons. This combined perspective will help scientists learn more about what to expect when imaging planets around other stars with future telescopes.

“While we expected Uranus to appear differently in each filter of the observations, we found that Uranus was actually darker than expected in the New Horizons data taken from a different perspective,” said l Lead author Samantha Hasler of the Massachusetts Institute of Technology in Cambridge and a collaborator on the New Horizons science team.

Direct imaging of exoplanets is a key technique for learning about their potential habitability and offers new clues about the origin and formation of our own solar system. Astronomers use both direct imaging and spectroscopy to collect light from the observed planet and compare its brightness at different wavelengths.

However, imaging exoplanets is a notoriously difficult process because they are so far away. Their images are only pinpoints and therefore are not as detailed as the close-up views we have of the worlds orbiting our sun. Researchers can also only directly image exoplanets at “partial phases,” when only part of the planet is illuminated by their star as seen from Earth.

Uranus was an ideal target as a test to understand future distant observations of exoplanets by other telescopes for several reasons. First, many known exoplanets are also gas giants of a similar nature. In addition, at the time of the observations, New Horizons was on the other side of Uranus, 6.5 billion kilometers away, which made it possible to study its twilight crescent, which cannot be done from Earth. At that distance, New Horizons’ view of the planet was just a few pixels in its color camera, called the Multispectral Visible Imaging Camera.

On the other hand, Hubble, thanks to its high resolution and low Earth orbit 1.7 billion kilometers from Uranus, was able to see atmospheric features such as clouds and storms on the day side of the world gaseous.

“Uranus appears as a small dot on New Horizons observations, similar to dots seen on exoplanets directly imaged by observatories like Webb or ground-based observatories,” Hasler added. “Hubble provides context for what the atmosphere is doing when observed with New Horizons.”

The gas giant planets in our solar system have dynamic and variable atmospheres with changing cloud cover. How common is this among exoplanets? By knowing the details of how clouds appear on Uranus from Hubble, researchers are able to verify what is being interpreted from the New Horizons data. In the case of Uranus, Hubble and New Horizons found that the brightness did not change as the planet rotated, indicating that the characteristics of the clouds did not change as the planet rotated.

However, the importance of New Horizons’ detection relates to how the planet reflects light at a different phase than Hubble, or other observatories on or near Earth, can see. New Horizons showed that exoplanets can be darker than expected at partial and high phase angles, and that the atmosphere reflects light differently at partial phase.

NASA has two major observatories in the works to advance studies of exoplanet atmospheres and their potential habitability.

“These historic New Horizons studies of Uranus from a vantage point unobservable by any other means add to the mission’s trove of new scientific knowledge and have, like many other datasets obtained during the mission, provided surprising new information about the worlds of our solar planet system,” added New Horizons principal investigator Alan Stern of the Southwest Research Institute.

NASA’s upcoming Nancy Grace Roman Space Telescope, scheduled to launch by 2027, will use a coronagraph to block the light of a star to directly view gas giant exoplanets. NASA’s Habitable Worlds Observatory, in its first planning phase, will be the first telescope designed specifically to search for atmospheric biosignatures on rocky, Earth-sized planets orbiting other stars.

“Studying how known references like Uranus appear in distant imagery can help us have more robust expectations when preparing for these future missions,” Hasler concluded. “And that will be critical to our success.”

Launched in January 2006, New Horizons made the historic flyby of Pluto and its moons in July 2015, before giving humanity its first close-up look at one of these planetary building blocks and Kuiper Belt object, Arrokoth, in January 2019. New Horizons is now on its second extended mission, studying distant Kuiper Belt objects, characterizing the sun’s outer heliosphere, and making important astrophysical observations from its unparalleled vantage point in remote regions of the solar system.

The Uranus results are being presented this week at the 56th annual meeting of the Division of Planetary Sciences of the American Astronomical Society, in Boise, Idaho.