Exploring exoplanets – planets outside of our solar system – could help scientists answer big questions about our place in the universe, and if life exists beyond the Earth. But these distant worlds are extremely weak and hard to visualize directly. A new study uses Earth as a substitute for an exoplanet and shows that even with very little light – as little as one pixel – it is still possible to measure key features of distant worlds.
The new study uses data from NASA's EPIC instrument aboard the Deep Ocean Climate Observatory of the National Oceanic and Atmospheric Administration (DSCOVR). DSCOVR orbits the Sun at Lagrange Point 1, a specific orbit that provides EPIC with a constant view of the sunlit surface of our home planet. EPIC has been observing the Earth continuously since June 2015, producing nuanced maps of the planetary surface in multiple wavelengths and contributing to climate and weather studies.
The EPIC instrument captures reflected light from Earth in 10 different wavelengths or colors. Every time EPIC takes a picture of the Earth, it actually captures 10 pictures. The new study averages each image into a single brightness value or the equivalent of a "one-pixel" image for each wavelength. A single one-pixel snapshot of the planet would provide very little information about the surface. In the new study, however, the authors analyzed a data set of individual images taken several times a day in 10 wavelengths over a longer period of time. Despite the fact that the entire planet was reduced to a single point of light, the authors were able to identify water clouds in the atmosphere and measure the rotation speed of the planet (the length of its day). The authors state that the study in the April 27 issue Astrophysical Journal shows that the same information can be derived from one-pixel observations of exoplanets.
"The benefit of using Earth as a proxy for an exoplanet is that we can validate our conclusions from the one-pixel data with the wealth of data we actually have for the Earth – we can not do that, though We use data from a distant, actual exoplanet, "said Jonathan Jiang, atmospheric and climate researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, and lead author of the new study.
A Tiny Spot
When Jiang's daughter Teresa was in elementary school, he organized a star-gazing event for her and her friends. Jiang pointed to the stars and told his daughter that the sun is also a star, and that there are planets orbiting other stars as planets orbit the sun. She asked her father for more information and asked how scientists could possibly learn about these distant worlds from such tiny points of light in the sky.
"Children ask a lot of good questions," Jiang said. "And I remembered that question – if I can see an exoplanet as a tiny point of light, can I see clouds and oceans and land?"
Jiang began his career in astrophysics, but for his Ph.D. Work he decided to apply his computer and physical modeling skills to the Earth's climate. Now he uses climate data to support the study of exoplanets. Exoplanets are much darker than stars and much harder to spot. Earth, for example, is about 10 billion times weaker than the Sun. Only about 45 exoplanets were discovered by direct imaging, all much larger than Earth. The majority of known exoplanets (over 3,700 confirmed) have been indirectly detected using techniques such as the transit method, in which scientists observe the slight darkening of a star caused by the transit of an exoplanet over the face of the star.] NASA uses the Earth as a laboratory to study distant worlds “/>
The EPIC instrument captures reflected light from the sunlit side of the earth in 10 different wavelengths or colors, as different materials reflect different wavelengths of light to different degrees – plants, for example, mainly reflect green light. And a reddish planet like Mars, for example, would have a completely different color profile than a planet covered in ice.
The new study shows that a planet with certain features is observed over time – such as oceans and continents. It is possible to measure the rotational velocity of the planet by observing a repeating pattern in the reflected light. This pattern would result from those planetary features that come in sight with a regular cadence. For example, Australia and the Pacific fill the field of view of the EPIC every 24 hours, and about 12 hours later, South America and the Atlantic Ocean fill the border between Africa and the Indian Ocean. This pattern of changing light would be repeated day after day. In the new work, the authors show that they can recognize this repetitive cycle and thus determine the rotation rate or the length of the planet. The rotational speed of a planet can provide information about how and when the planet has formed, and is a particularly difficult property that can be measured using current methods.
"People have been talking for some time about using this method of measuring rotational velocity in exoplanets, but there was no demonstration that it could work because we did not have real data," said Renyu Hu, an exoplanet scientist on the JPL and co-author of the new study. "We have shown that at every wavelength the 24-hour period occurs, which means that this approach to measuring planetary rotation is robust."
The authors note, however, that the effectiveness of this method depends on the unique characteristics of the planet. A pattern of the daily cycle may not be visible on a planet that is largely homogeneous on its surface. For example, Venus is covered by thick clouds and has no oceans on its surface, so a recurring, monotonous pattern may not or perhaps not clearly enough to be seen in a one-pixel image. Planets like Mercury and Mars would also be a challenge, but Jiang said planetary features such as craters could also contribute to a pattern that could be used to measure the rotation period.
Earlier studies used Earth as a substitute for exoplanets to investigate which types of planetary properties can be derived remotely, but no previous study looked at so many wavelength ranges. This is also the first such study to cover such a large dataset over a longer period of time: more than 27 months of observation were used, with EPIC taken about 13 times a day.
Direct observations of exoplanets have much less data than those used in the new study, but the researchers report that measuring the rotational velocity of an exoplanet with more than 90 percent confidence is a record of only two to three times per round trip period would require (ie per "day" on this particular exoplanet) for approximately seven round trip times.
How long astronomers must observe an exoplanet to determine its rotational speed also depends on how much unwanted light is contained in the exoplanet data. The EPIC data provide an exceptionally clear view of the earth, largely independent of light from other sources. However, a primary challenge in direct imaging of exoplanets is that they are so much weaker than their parent stars. The light of the near star can easily drown out the light of an exoplanet and make the latter invisible. If the signal from the planet is competing with the star's light, it may take longer to detect a pattern that could reveal the planet's rotation speed. NASA is investigating possible designs for next-generation telescopes that could directly record Earth-like exoplanets.
With the field of direct imaging of exoplanets, Jiang is no longer concerned about the question his daughter posed more than a question a decade ago. If scientists can learn about the surface features of distant planets, they could then answer an even bigger question his daughter has asked – are these planets doing life?
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