While the Earth absorbs a lot of energy from the sun, much of it is reflected back into space. The sunlight reflected from the earth is called the sunshine. We can see it on the dark part of the moon during the crescent moon. The Farmer’s Almanac said it was called “New Moon in the Arms of Old Moon.”
The brightness of the Earth is one example of planetary brilliance, and when we look at the light from distant exoplanets, we are looking directly at the brightness of the planets without bouncing off another object.
If distant astronomers view the brightness of Earth the way we view the brilliance of exoplanets, does the light tell them that our planet is teeming with life?
In the next few years, a number of advanced telescopes will be brought online. Together with the JWST, they will give us the kinds of images that scientists have been eagerly anticipating for decades. Thanks to the ground-based European Very Large Telescope and the Giant Magellan Telescope, and the upcoming LUVOIR space telescope, we will enter the era of live imaged exoplanets. Scientists need to prepare for all of these observations and data in order to be ready to interpret them.
These future telescopes will allow astronomers to characterize more and more Earth-like exoplanets, we hope. But the only way we describe these planets It can be accurate if our models are accurate. Since Earth is the only planet we know of to host life and the only habitable planet with known characteristics, it is our only test case and the only resource for astronomers to validate their models.
This is where Earthshine comes in.
In a new paper, a team of researchers investigates how Earth’s radiance can be used to build accurate models of planetary rays. The paper is “Polarized Signatures of a Habitable World: Comparison of Exoplanet Models with Visible and Near-Infrared Luminescence Spectra.” The lead author is Kenneth Gordon, a graduate student in the Planetary Science Group at the University of Central Florida. The paper was accepted Astrophysical Journal.
We are discovering an increasing number of rocky planets in potentially habitable zones around exoplanets. But to get closer to understanding whether they are habitable, we need to describe their surfaces. Astronomers have limited tools to do this, often by studying the light from planets as they pass in front of their star or detecting the flux directly from the planet.
These methods work for large gaseous planets. But it’s hard on rocky planets, and rocky planets are what we care about. Large gaseous planets have puffy atmospheres that make spectral study easier. And they emit or reflect more light due to their size, which gives them a higher flux in direct imaging. But rocky planets have much smaller atmospheres, which makes them more difficult to study using spectroscopy. Because they are smaller, their flow is also lower, making them more difficult to shoot directly.
As our telescopes get more powerful, they will overcome some of these hurdles to characterize rocky exoplanets. This new paper is part of how the astronomy community is preparing.
In their paper, the authors point out how the strong co-planet theory of Earth hinders its efforts to fully characterize Earth-like exoplanets. Characterizing the atmospheres of these planets around cold dwarf stars requires long periods of observation. In a previous paper, a separate team of researchers showed that JWST would need to monitor more than 60 transits of one of the well-known TRAPPIST-1 rocky exoplanets to detect Earth-like ozone levels.
Using JWST’s Near Infrared Spectrometer (NIRSpec) and Medium Infrared Instrument (MIRI), they found that >60 transits for 1b and >30 transits for 1c and 1d would be required to detect Earth’s current levels of ozone (O).3) on these planets,” the authors write.
JWST will also grapple with what astronomers call decay. “…a number of aberrations will still be present in characterizations of habitable worlds by JWST, such as the distinction between optical thickness and particle size distributions of clouds,” they wrote.
The researchers focus on measuring polarization in their work. In short, polarimetry is the measurement of polarized light that has been affected in some way by the materials through which it passes, is reflected, refracted or deflected by it. Polarimetry is also the interpretation of measurements.
Measuring polarization could be the key to breaking the ice between our advanced telescopes and the small, rocky planets we want to study. It can reduce the monitoring time required as well. “Polarimetry is a powerful technique that has the potential to break down these aberrations because it evaluates the physical aspects of light not measured in non-polarization photometry or spectroscopy.”
The polarimetry is powerful because it is very sensitive to the properties of the exoplanet’s atmosphere. It has proven effective in studying our solar system, including Venus, which is surrounded by clouds. “Polarization measurement has helped characterize objects in the solar system, including the clouds of Venus and gas giant planets, as well as the various icy conditions of the Galilean moons,” the authors explain. Polarimetry has been so effective in studying Venus that some want to build a polar radar to study the planet more fully.
The problem is that astronomers don’t have accurate polar models of exoplanets to help them make sense of what they’re seeing when they study planets with polar poles. Models do exist, but they need to be tested and validated against real planets, and that’s where Earth comes in. “To date, Earth is the only known and observed ‘Earth-like’ planet, and thus serves as a standard for inferring biosignatures of life as we know it today.”
According to the researchers, the brightening of the Earth is the key to this. Optical and infrared (NIR) luminous flux spectra studies reveal bio-diagnostic signatures of Earth, including vegetation red edge (VRE), ocean luster, and atmospheric O spectral features.2 and h2O.” Other studies have also shown what polarimetry can make an effective contribution to these observations.
Light reflecting off the Earth is polarized, but after bouncing off the Moon, it becomes polarized. The authors corrected this in their work. They studied five different types of planetary surfaces under clear skies and cloudy skies. They also looked at different types of clouds with different particle sizes.
The main point of the study was to compare two different existing models that astronomers can use to interpret polarimetry and measure their accuracy. One is called DAP and the other is called VSTAR. The team used both to interpret and then compare their polarimetry data.
This type of research shows how much work goes into a scientific endeavor. While astronomy titles may make things seem simple, they are complex. There is much more to it than just pointing powerful telescopes at distant objects and then looking at the images. It takes a dedicated effort of thousands of people over decades to make astronomy work. There’s a lot at stake, and if one day a team of astronomers says, “We’ve done it! We’ve discovered a planet with life!” It would be because a work as detailed and intricate as this one doesn’t generate much headlines.
Kenneth Gordon et al., Polarizing signatures of a habitable world: Comparing models of an exoplanet with visible and near-infrared fluorescence spectra, arXiv (2023). doi: 10.48550/arxiv.2301.05734
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