Is the discovery of oceans on planets orbiting distant stars within our reach? Finding such an ocean would be of immense interest from an astrobiological perspective because water on the surface is the traditional marker for a habitable zone. Astrobiology Magazine has just written up work by Nicholas Cowan (Northwestern University) and colleagues, who have been looking at the ways we might detect such oceans. The researchers are thinking ahead to a time when we have an actual image of a terrestrial world to look at, even if that image is little more than the ‘pale blue dot’ Voyager saw in its famous portrait of the Solar System. When we have identified that ‘dot,’ we can do a lot with it by studying the way its light varies as it orbits its star.
Let’s assume we deploy a starshade and use it in conjunction with the James Webb Space Telescope to block the light of the star and reveal the faint signature of the planet. A disk tens of meters wide with petal-like extensions, the starshade would be placed between the telescope and the star under observation, its shape designed to prevent the rings and refractions that would be created by a circularly shaped shade. One option under consideration is to place the starshade about 160,000 kilometers away from the telescope, which will orbit at the L2 Lagrangian point. Such a configuration could yield the image of a terrestrial world in the habitable zone, a planet whose variations in light can tell us something about what is on its surface.
Finding oceans then becomes a major first step in characterizing the planet. The Cowan paper presents the three methods that have so far been proposed for detecting alien oceans:
- Changes in color
Variations in the color of the planet are useful because oceans are darker and have different colors than the surface features of continents. Watching the planet over time should reveal these changes.
- Polarized light
Oceans polarize light, whose phase variations can flag the presence of water. The trick is that light also scatters off molecules in the atmosphere (‘Rayleigh scattering’), masking the effect, but rotational variation in polarization may allow us to infer the presence of an ocean.
- Specular reflection
Oceans can throw bright reflections, especially when the planet is in its crescent phase, making the planet appear brighter than would otherwise be expected. Variations in reflectivity (albedo) as the planet circles its star can thus be markers for an ocean if properly interpreted.
Image: Glinting sunlight off Lake Erie. Can we use this kind of specular reflection to identify the oceans of an alien world? Credit: Image Science and Analysis Laboratory/NASA JSC.
Cowan and team focus largely on the specular reflection method, noting that all three techniques have been studied in terms of cloud cover and changes in albedo due to seasonal changes on the surface. But they also identify what they call the ‘latitude-albedo’ effect, which can play havoc with these observations by mimicking the glint of an ocean when none actually exists. The reason: A planet in the habitable zone with low axial tilt (obliquity) would tend to have highly reflective snow and ice in the regions least illuminated by sunlight. A false positive for ocean glint is thus produced.
In other words, the polar regions will make the apparent reflectivity of a planet with low axial tilt increase when the planet is seen in its crescent phase, an effect that will diminish in the gibbous phase. The latitude-albedo effect thus limits our ability to use ocean ‘glint’ as a marker for water on the surface, though the authors note there are some ways around the problem. It will be necessary to study the color variations of the planet during its own rotation and during the entirety of its orbit to develop an estimate for the planet’s obliquity. The rotational albedo map that can be generated out of this should allow better interpretation of the variations in light observed. The JWST/starshade combination may be powerful enough to monitor these tiny changes.
If you’re wondering how significant the ‘glint’ effect could be, consider that Tyler Robinson (University of Washington), modeling the Earth as it would appear to a distant observer back in 2010, was able to show that the Earth would be as much as 100 percent brighter at crescent phases when modeled with the glint effect than without it. Thus specular reflection can be a major player in characterizing an exoplanet, but only if we learn how to interpret it properly.
The Cowan team’s simulation worked with a planet whose obliquity is 23.5 degrees, the same as the Earth’s, calculating light curves as they would appear to a distant observer. Subtracting out the kind of reflection that would produce an ocean glint, they still found the false positive, phase variations that mimic the glint. Planets like the Earth have enough axial tilt that the methods above can correct for the latitude-albedo effect, but the authors note that zero-obliquity planets will be extremely hard to investigate. It’s worth noting that planets like these should be fairly common around red dwarf stars, where the planet has become tidally locked to the primary.
The paper is Cowan, Abbot and Voigt, “A False Positive For Ocean Glint on Exoplanets: the Latitude-Albedo Effect,” accepted at Astrophysical Journal Letters (preprint). Thanks to Antonio Tavani for the pointer to this paper.