The study of exoplanet planets outside our solar system – can help researchers answer major questions about our place in the universe and if life is outside the world. But these distant worlds are extremely weak and difficult to picture directly. A new study uses the Earth as a stand-in for an exoplanet, and shows that even with very little light – as little as a pixel – it is still possible to measure important characteristics of distant worlds.
The new study uses data from NASA's Earth Polychromatic Imaging Camera (EPIC) instrument, which aboard the US Pacific Ocean's atmospheric management's Deep Space Climate Observatory or DSCOVR satellite. DSCOVR walks around the sun in Lagrange Point 1, a specific orbital that gives EPIC a constant image of our home-style sunlit surface. EPIC has continuously observed the Earth since June 2015 and produces nuanced maps of the planet's surface at several wavelengths and contributes to climate and weather studies.
The prisoners of the EPIC instrument reflected light from the earth in 10 different wavelengths or colors. So, every time EPIC "takes a picture" of the earth, it actually captures 10 pictures. The new study estimates each image in a single brightness value, or equivalent to a "one pixel" image for each wavelength. A single, one-point snapshot of the planet would provide very little information about the surface. But in the new study, the authors analyzed a dataset containing single pixel images taken several times a day, in 10 wavelengths over a longer period of time. Although the whole planet had been reduced to a single point of light, the authors could identify water clouds in the atmosphere and measure the planet's rotational velocity (its length). The authors say that the study, in the June 27 issue of Astrophysical Journal shows that the same information can be derived from single pixel observations of exoplanets.
"The advantage of using the land as a proxy for an exoplanet is that we can verify our conclusions derived from single pixel data with the amount of data we actually have for the earth. We can not do that if we use data from a remote , actually exoplanet, says Jonathan Jiang, an atmospheric and climate researcher at NASA's Jet Propulsion Laboratory in Pasadena, California, and leading writer of the new study.
A small bright spot
When Jiang's daughter, Teresa, was in elementary school, he organized a star observation event for her and her friends. Jiang pointed to the stars and told his daughter that the sun is also the star, and that there are planets circling around other stars just as the planets circle the sun. She pressed her dad for more information and asked how researchers could learn about the distant worlds from such little light points in the sky.
"Children ask very good questions," said Jiang. "And that question stays in my mind – if I can see an exoplanet as just a small point of light, can I see clouds and sea and land?"
Jiang started his career in astrophysics, but for his doctoral work, he decided to apply his computer and physical modeling skills to the Earth's climate. Now he uses climate data to assist in studying exoplanets. Exoplanets are significantly dimmer than stars and much harder to detect. Soil is, for example, about 10 billion times weaker than the sun. Only about 45 exoplanets have been detected by direct formation, all of which are much larger than the Earth. The majority of known exoplanets (over 3 700 have been confirmed) were indirectly detected using techniques such as the transit method, where researchers observe the weak diminishment of a star caused by an exoplanet's transit across the star's face.  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 of plants, for example, mostly reflecting green light. And, for example, a reddish planet like Mars would have a completely different color profile compared with an ice-covered plan.
The new study shows that by observing a planet with clear characteristics over time – like oceans and continents – it is possible to measure the planet's rotational velocity by observing a repeating pattern in the reflected light. This pattern should arise from the planetary features moving in the direction of a common cadence. For example, Australia and the Pacific Ocean fill every day EPIC's field of view, and about 12 hours later, South America and the Atlantic fill up the framework, with Africa and the Indian Ocean passing between them. This pattern of change of light would be repeated day by day. In the new paper, the authors show that they can detect this repetitive cycle and thus determine the rotation speed or the planet's length. The rotation speed of a planet can reveal information about how and when the planet was formed and is a particularly difficult feature to measure with current methods.
"People have talked about using this method to measure the rotational speed of exoplanets, but it has not been shown that it could work because we had no real data," said Renyu Hu, an exoplanet researcher at JPL and a co-author on the new study. "We have shown that in every wavelength the 24-hour period appears, which means that this way of measuring planet rotation is robust."
However, the authors note that the effectiveness of this method would depend on the unique characteristics of the planet. A daily bicycle pattern may not be visible on a planet that is largely homogeneous across its surface. For example, Venus is covered in thick clouds and has no oceans on its surface, so a recurring long-lasting pattern may not appear or may not be sufficiently clear for to observe in a one-pixel image. Planets like mercury and Mars would also be challenging, but Jiang said that the planet's trait Being like craters could also contribute to a pattern that can be used to measure the rotation period.
Previous studies used the Earth as a proxy for exoplanets, to investigate what types of planetary properties could be distracted far away, but no previous studies looked at so many wavelength bands. This is also the first study to capture such a large dataset, over a longer period of time: it used more than 27 months of observations, with pictures taken by EPIC 13 times a day.
Direct observations of exoplanets have much less data than were used in the new study, but the researchers report that to measure the rotational speed of an exoplanet by more than 90 percent, confidence would require taking pictures only two to three times per orbital period (that is, per "
The number of astronomers would have to observe an exoplanet to identify its rotation speed also depends on how much unwanted light is included in exoplanet data. EPIC data provides an extremely clear image of the earth, largely unprotected by light from other sources. But a primary challenge for direct images of exoplanets is that they are so much dimmier than their parent stars. The light from the nearby star can easily drown the light from an exoplanet, making it later invisible. With the signal from the planet competing with light from the star, it may take longer to distinguish a pattern that can reveal the planet's rotational speed. NASA examines potential patterns for next generation telescopes that can directly form exoplanets from Earth's size.
With the exoplan's direct imaging, Jiang continues to not think about the question that his daughter asked him more than a year ago. If researchers can learn about the surface of distant planets, they can respond to an even bigger issue that his daughter posed – do any of these planets host life?
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