Astronomers used Hubble and New Horizons for a simultaneous observation of Uranus to help search for exoplanets.
Uranus has been described as mysterious, strange and quite unknown to us on Earth, however in astronomy these terms are quite relative. Compared to remote, primordial or wacky exoplanets dozens of light-years from our solar system, researchers actually know a lot about Uranus. NASA’s Hubble Space Telescope is one of the observatories capable of observing the planet in high resolution, showing close-up details of the planet’s atmospheric characteristics. Astronomers have now exploited this view in a new way. They have with Hubble and, at the same time, with the New Horizons space probe from 6.5 billion miles away, where the planet appears only as a speck. This combined perspective can help learn more about what to expect when imaging exoplanets around other stars: Hubble provides context for what the atmosphere is actually doing when observed with New Horizons.
Towards understanding exoplanets
Astronomers used Uranus as a proxy for beyond our Solar System by comparing high-resolution images from Hubble with 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.
“We expected Uranus to appear differently in each filter of the observations, but we found that it was actually fainter than expected in New Horizons data taken from a different vantage point,” said lead author Samantha Hasler of Massachusetts Institute of Technology in Cambridge and collaborator of the New Horizons scientific 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 solar system. Astronomers use it 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 point-like and therefore are not as detailed as the close-up views we have of worlds orbiting our Sun. Researchers can also only obtain direct images of exoplanets in “partial phases,” when only a portion of the planet is illuminated from their star as seen from Earth.
Uranus was an ideal test target for understanding future distant observations of exoplanets by other telescopes for a few reasons. First, many known exoplanets are also similar in nature. Additionally, at the time of the observations, New Horizons was on the far side of Uranus, 6.5 billion miles away, allowing it to study its twilight crescent, something that cannot be done from Earth. At that distance, New Horizons’ view of the planet was only several pixels in its color camera, called the Multispectral Visible Imaging Camera.
Precious help from Hubble
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On the other hand, thanks to its high resolution and its low Earth orbit, 2.7 billion kilometers away from Uranus, it was able to observe atmospheric features such as clouds and storms on the illuminated side of the gaseous world. “Uranus appears only as a small dot in New Horizons observations, similar to dots seen on exoplanets imaged directly by observatories like Webb or by ground-based observatories,” Hasler added. “Hubble provides context for what the atmosphere is doing when it was observed with New Horizons.”
The gas giant planets in our solar system have dynamic, variable atmospheres with changing cloud cover. How common is this among exoplanets? By knowing the details of 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, indicating that the characteristics of the clouds did not change as the planet rotated.
However, the importance of New Horizons’ detection has to do with 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 may be dimmer than expected at partial and high phase angles, and that the atmosphere reflects light differently at partial phase. NASA is working on two major observatories to advance studies of exoplanet atmospheres and their .
“These fundamental studies conducted by New Horizons of Uranus from a vantage point unobservable by other means add to the mission’s treasure trove of new scientific knowledge and, like many other datasets obtained throughout the mission, have provided surprising new insights into worlds in our solar system,” added Alan Stern of the Southwest Research Institute, principal investigator of New Horizons.
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Future telescopes
NASA’s next one, scheduled to launch in 2027, will use a coronagraph to block the light of a star and see gas giant exoplanets directly. NASA’s Habitable Worlds Observatory, in an early planning stage, will be the first telescope designed specifically to search for atmospheric biosignatures on Earth-sized rocky planets orbiting other stars.
“Studying how well-known landmarks like Uranus appear in distant images can help us have more solid expectations when preparing for these future missions,” Hasler concluded. “And that will be critical to our success.”
Launched in January 2006, New Horizons surveyed its moons in July 2015, before offering humanity its first close-up look at one of these planetary building blocks and Kuiper Belt objects, 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 unrivaled vantage point in distant regions of the solar system.
The Uranus results will be presented this week at the 56th annual meeting of the Division of Planetary Sciences of the American Astronomical Society, in Boise, Idaho.
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