In a new study, astronomers compared high-resolution images of Uranus from the NASA/ESA Hubble Space Telescope to the more-distant view from NASA’s New Horizons spacecraft. Their results may serve as ‘ground-truth’ observations to use as a baseline to interpret exoplanet direct-imaging data from future observatories.
In this image, two three-dimensional shapes (top) of Uranus are compared to the actual views of the planet from Hubble (bottom left) and New Horizon (bottom right). Image credit: NASA / ESA / STScI / Samantha Hasler, MIT / Amy Simon, NASA-GSFC / New Horizons Planetary Science Theme Team / Joseph DePasquale, STScI / Joseph Olmsted, STScI.
Direct imaging of exoplanets is a key technique for learning about their potential habitability, and offers new clues to 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’re so far away.
Their images are mere pinpoints and so are not as detailed as the close-up views that we have of worlds orbiting our Sun.
Astronomers can also only directly image exoplanets at ‘partial phases,’ when only a portion of the planet is illuminated by their star as seen from Earth.
Uranus was an ideal target as a test for understanding future distant observations of exoplanets by other telescopes for a few reasons.
First, many known exoplanets are also gas giants similar in nature. Also, at the time of the observations, New Horizons was on the far side of Uranus, 10.5 billion km (6.5 billion miles) away, allowing its twilight crescent to be studied — something that cannot be done from Earth.
At that distance, the New Horizons view of the planet was just several pixels in its color camera — the Multispectral Visible Imaging Camera.
On the other hand, Hubble, with its high resolution, and in its low-Earth orbit 2.7 billion km (1.7 billion miles) away from Uranus, was able to see atmospheric features such as clouds and storms on the day side of the gaseous world.
“While we expected Uranus to appear differently in each filter of the observations, we found that Uranus was actually dimmer than predicted in the New Horizons data taken from a different viewpoint,” said MIT astronomer Samantha Hasler.
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 what the clouds on Uranus looked like from Hubble, researchers are able to verify what is interpreted from the New Horizons data.
In the case of Uranus, both Hubble and New Horizons saw that the brightness did not vary as the planet rotated, which indicates that the cloud features were not changing with the planet’s rotation.
However, the importance of the detection by New Horizons has to do with how the planet reflects light at a different phase than what Hubble, or other observatories on or near Earth, can see.
New Horizons showed that exoplanets may be dimmer than predicted at partial and high phase angles, and that the atmosphere reflects light differently at partial phase.
“These landmark New Horizons studies of Uranus from a vantage point unobservable by any other means add to the mission’s treasure trove of new scientific knowledge, and have, like many other datasets obtained in the mission, yielded surprising new insights into the worlds of our Solar System,” said New Horizons principal investigator Dr. Alan Stern, a researcher at the Southwest Research Institute.
“NASA’s upcoming Nancy Grace Roman Space Telescope, set to launch by 2027, will use a coronagraph to block out a star’s light to directly see gas giant exoplanets,” Dr. Hasler said.
“NASA’s Habitable Worlds Observatory, in an early planning phase, will be the first telescope designed specifically to search for atmospheric biosignatures on Earth-sized, rocky planets orbiting other stars.”
“Studying how known benchmarks like Uranus appear in distant imaging can help us have more robust expectations when preparing for these future missions. And that will be critical to our success.”
The scientists presented the results this week at DPS56, the American Astronomical Society’s Division for Planetary Sciences annual meeting.
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S. Hasler et al. 2024. New Horizons Ralph/MVIC Observations of Uranus at High Phase Angles. DPS56
This article was adapted from an original release by NASA.
