Astrobiology
The Earth as a Planet
Our research team has consistently pursued a pioneering philosophy: using our own Earth as the ultimate Rosetta Stone to decode the mysteries of distant worlds.
Before we could accurately characterize the chemical composition of planets orbiting other stars, we recognized that we first needed to understand how a life-bearing planet like ours appears when viewed from a cosmic distance.
In our comprehensive book, The Earth as a distant planet, we try to bridge the gap between Earth-based observations, climate science and astrobiology, and we established the essential benchmarks for the future search for biological signatures across the galaxy.
The Earthshine Project
The Earthshine Project, conducted primarily at Big Bear Solar Observatory (BBSO) in collaboration with the California Institute of Technology and the Instituto de Astrofísica de Canarias, is a long-term scientific endeavor designed to precisely measure Earth’s global albedo (the fraction of sunlight our planet reflects into space) .
The project’s methodology leverages the phenomenon of earthshine, often called the «Da Vinci glow», which is sunlight that reflects off Earth, travels to the Moon, and bounces back, creating the faint glow visible on the lunar dark side. Using specialized photometric telescopes at BBSO and a calibrated twin instrument in Tenerife for global longitudinal coverage, the project measures the brightness ratio between the dark (earthshine-lit) and bright (directly sunlit) portions of the Moon . This technique provides an integrated measurement of Earth’s large-scale reflectance, providing an alternative to traditional satellite instruments and the advantage of an absolute calibration.
The project pursues dual scientific goals with profound implications. First, it aims to characterize the synoptic, seasonal, and inter-annual variability of Earth’s albedo to improve climate models, challenging the traditional assumption that this parameter is roughly constant. Second, it serves an astrobiological purpose by using Earth as a proxy for an exoplanet; spectroscopic observations of earthshine provide a template for what the reflected light from a distant, potentially habitable world might look like, informing the search for life beyond our solar system.
A landmark result from the project is the 2004 Science paper, titled «Changes in Earth’s Reflectance over the Past Two Decades«, where by correlating earthshine data with satellite cloud observations, we found a steady, climatologically significant decrease in Earth’s reflectance from 1984 to 2000, followed by a reversal after 2001. This finding revealed unexpected decadal-scale variability in Earth’s albedo linked to cloud changes, highlighting critical complexities in understanding climate forcing and feedbacks.
In Montañés-Rodríguez et al, we used earthshine observations from Palomar Observatory, satellite cloud data from the International Satellite Cloud Climatology Project (ISCCP), and radiative transfer modeling to assess the detectability of the vegetation red edge in Earth’s globally integrated spectrum. The researchers found a strong correlation between red edge intensity and cloud-free vegetated area. They concluded that the red edge signal in globally averaged spectra is weak and only attributable to vegetation when actual land and cloud distributions are known, though it becomes prominent under certain orbital geometries. Their work demonstrated that vegetation detection on Earth-like exoplanets, while difficult, is a possibility.
But the Earth’s reflectance spectrum has changed over time, specially over very long time scales. In Sanromá et al, we modeled the globally integrated spectrum of the Archean Earth (3 billion years ago), when purple bacteria were among the most widespread photosynthetic life forms. Using a radiative transfer model, we simulated how different distributions of these bacteria over land and oceans would affect Earth’s disk-averaged reflectance. Purple bacteria exhibit a strong reflectivity increase in the near-infrared, analogous to the red edge of modern vegetation but shifted to longer wavelengths . This feature produces a detectable signal depending on cloud cover and bacterial concentration. We concluded that multi-color photometric observations could distinguish between a «purple Earth» dominated by these organisms and the present-day Earth.
We have also explored the use of spectropolarimetry of Earthshine to reveal unique biosignatures, such as the presence of vegetation, or strongly polarized band of water vapor in the near-infrared which might one day be used to characterize exoplanets in polarized light.
The Earth as a Transiting Planet
One of our most significant contributions to the field was providing the scientific community with a template for identifying habitability through the analysis of the Earth’s transmission spectrum during lunar eclipses.
This work, published in Nature (2009) by Pallé et al, utilized earthshine-style observations during the total lunar eclipse of August 2008, treating the Moon as a natural mirror reflecting sunlight transmitted through Earth’s atmosphere.
This geometry allowed us to measure the transmission spectrum of Earth’s limb as if observing a transiting exoplanet. Our analysis revealed the presence of ozone, oxygen, and water vapor, demonstrating the simultaneous detection of biological markers. The work provided a crucial proof-of-concept for characterizing potentially habitable exoplanets through transmission spectroscopy during transits. It demonstrated that the same chemical signatures we see in our own sky could be detected in the filtered starlight of an exoplanet.
The Solar System Planets as Exoplanets
Observing the Earth as a planet is a technique that can be also exported to other solar system bodies.
In Montañés-Rodríguez et al we observed Ganymede transiting Jupiter’s shadow, effectively obtaining Jupiter’s transmission spectrum as if it were a transiting exoplanet. Using UV-to-NIR observations from the Very Large Telescope, they revealed strong extinction from hazes and aerosols, prominent methane absorption, and—critically—spectral signatures of crystalline water ice at 1.5 and 2.0 microns, indicating a stratospheric water-ice cloud layer. We also detected sodium absorption in Jupiter’s upper atmosphere. This work validated transmission spectroscopy techniques for exoplanet characterization while providing new insights into Jupiter’s atmospheric composition seen as an exoplanet.
Main Projects
SPEAR
Exoplanet Atmospheres
Astrobiology
MuSCAT2
Space Missions
Collaborations
Deciphering the atmospheres of distant worlds
“Funded by the European Union. Views and opinions expressed are however those of the author(s) only and do not necessarily reflect those of the European Union or European Research Council Executive Agency (ERCEA). Neither the European Union nor the granting authority can be held responsible for them."