The study of exoplanets -- planets that lie outside our solar system -- could help scientists answer big questions about our place in the universe, and whether life exists beyond Earth. But, these distant worlds are extremely faint and difficult to image directly. A new study uses Earth as a stand-in for an exoplanet, and shows that even with very little light -- as little as one pixel -- it is still possible to measure key characteristics of distant worlds.
The new study uses data from NASA's Earth Polychromatic Imaging Camera (EPIC) instrument, which is onboard the National Oceanic and Atmospheric Administration's Deep Space Climate Observatory, or DSCOVR, satellite. DSCOVR goes around the Sun at Lagrange point 1, a specific orbit that provides EPIC with a constant view of our home planet's sunlit surface. EPIC has been observing Earth continuously since June 2015, producing nuanced maps of the planet's surface in multiple wavelengths, and contributing to studies of climate and weather.
The EPIC instrument captures reflected light from Earth in 10 different wavelengths, or colours. So, each time EPIC "takes a picture" of Earth, it actually captures 10 images. The new study averages each image into a single brightness value, or the equivalent of one "single-pixel" image for each wavelength. A single, one-pixel snapshot of the planet would provide very little information about the surface. But in the new study, the researchers analysed a data set containing single-pixel images taken multiple times per day, in 10 wavelengths, over an extended period. Despite the fact that the entire planet had been reduced to a single point of light, the researchers were able to identify water clouds in the atmosphere and measure the planet's rotation rate (the length of its day).
"The benefit of using Earth as a proxy for an exoplanet is that we can verify our conclusions derived from the single-pixel data with the wealth of data that we actually have for Earth -- we can't do that if we're using data from a distant, actual exoplanet," said Jonathan Jiang, an atmospheric and climate scientist at NASA's Jet Propulsion Laboratory in Pasadena, California, and lead author on the new study.
The EPIC instrument captures reflected light from the sunlit side of Earth in 10 different wavelengths, or colours, because different materials reflect different wavelengths of light to different degrees -- plants, for example, reflect mostly green light. And a reddish planet like Mars, for example, would have a very different colour profile compared to a planet covered in ice.
The new study shows that by observing a planet with distinct features over time -- such as oceans and continents -- it is possible to measure the planet's rotation rate by observing a repeating pattern in the reflected light. The researchers believe they can detect this repeating cycle and thus determine the rotation rate, or the length of the planet's day. The rotation rate of a planet can reveal information about how and when the planet formed, and is a particularly difficult property to measure with current methods.
The researcher observed, however, that the effectiveness of this method would depend on the unique features of the planet. A daily-cycle pattern might not be visible on a planet that is largely homogenous over its surface.
Previous studies used Earth as a proxy for exoplanets, to investigate what kinds of planetary properties could be derived from afar, but no previous studies looked at so many wavelength bands. This is also the first such study to capture such a large data set, taken over an extended period of time: it used more than 27 months of observations, with images taken by EPIC about 13 times per day.
But one primary challenge in directly imaging exoplanets is that they are so much dimmer than their parent stars. The light from the nearby star can easily drown out the light from an exoplanet, rendering the latter invisible. With the signal from the planet competing with light from the star, it may take longer to discern a pattern that could reveal the planet's rotation rate.
With the field of exoplanet direct imaging moving ahead, Jiang is not done thinking about the question his daughter asked him more than a decade ago. If scientists can learn about the surface features of distant planets, then could they answer an even bigger question that his daughter posed -- do any of those planets host life?