The Shade of Liveable Worlds

The Color of Habitable Worlds

“Here we live. On a blue point. "said Carl Sagan when the now famous Pale Blue Dot photo was released. Pale Blue Dot was captured by the Voyager 1 spacecraft on February 14, 1990 and, at 6 billion kilometers, is the farthest photo on Earth ever taken Last February, Pale Blue Dot celebrated its 30th anniversary, which was reprocessed with modern digital photo techniques to create an even more remarkable image.

This updated version of the iconic Pale Blue Dot image captured by the Voyager 1 starship uses modern image processing software and techniques to revisit the familiar Voyager view while trying to respect the original dates and intentions of those who planned the pictures.
Photo credit: NASA / JPL-Caltech

Whether pale blue dot or blue marble, our planet is associated with the color blue. Since Earth is the only inhabited world we know, it might be obvious that other habitable planets in space will also be blue. But it's a little more complicated.

What is color

By “color”, astronomers mean the intensity of light at a certain wavelength. Light, which is electromagnetic (EM) radiation, moves through space like a wave, just like waves move through water. The length of this wave determines its color. For example, EM radiation appears as the color blue to our eyes at wavelengths around 450 nanometers. What we humans generally consider color, however, only makes up a small fraction of the total wavelengths of the EM spectrum. Telescopes can perceive different parts of the spectrum outside of what our eyes see, such as B. Ultraviolet or Infrared, which can also be viewed as "color". The night sky would be much brighter to our eyes if we could see the full range of the EM spectrum. The upcoming James Webb Space Telescope can capture the infrared portion of the EM spectrum. Infrared penetrates interstellar dust and gas more easily than visible light, allowing James Webb to see through obstacles. This is where it gets exciting – color can actually tell us a lot about an object. The color of a star relates to its surface temperature. Red stars are cooler while blue stars are hotter. Color also tells us what something is made of. The color of an atmosphere indicates what is in the air. The color that is reflected from a surface tells us what that surface is. The color that comes from an object in the room is usually a mixture of several colors. The earth is more than just blue, it is a mixture of different colors, each representing different surfaces and gases on our planet. Every surface or atmospheric gas leaves its own “signature” on the wavelength of the sunlight that hits it, which leads to a change in color. When white sunlight falls on a plant, the chlorophyll absorbs some of the sunlight for energy, but reflects green and infrared light back into space. The science of this interaction between light and matter is called spectroscopy.

Colors and reflectivity of different earth surfaces c. NASA / Jeannie AllenThe EM Spectrum – Note that visible light that we “see” with our eyes is a very narrow part of the spectrum.

What about a distant world – an exoplanet – around another star? The color of a distant world can tell us about its habitability. Since it is not possible to warp to these planets or to form hyperspace, we instead use information that they sent us at the speed of light. However, the spectrographic investigation of distant Earth-like exoplanets presents two key challenges. First, our current generation of telescopes does not have the resolution necessary to distinguish the light of an Earth-sized planet from that of its parent star over the great distances we observe (remember how small the Earth never looked at 6 billion kilometers They hundreds of trillions). The light of the planet and the star are blurred together. We know exoplanets are out there, how big they are, whether they are terrestrial worlds, and what types of stars they orbit, but we can see little else about them right now. Second, even if our telescopes are powerful enough to examine the light of a single planet closely, we don't have a color map to use to see what we're seeing out there – no point of reference. We don't really know what "Earth" would look like from hundreds of light years away orbiting another star. The earth itself would be a different color under a red or blue sun. Fortunately, both challenges are mastered.

Examples of future super / extreme telescope projects – Universe Today

A new generation of high-resolution telescopes is on the horizon. Space telescope missions such as James Webb, HabEx and LUVOIR; Just like land-based telescopes such as the Giant Magellan Telescope, they have the resolution to separate the light of a relatively small, faint planet from that of its titanically blazing parent star. In anticipation of this increased telescope performance, Jack Madden – Ph.D. The Cornell University astrophysics candidate created a color guide for Earth-like worlds orbiting other stars. This guide, created using computer simulations, can be used to interpret the colors we see from distant worlds to determine if they are possibly habitable.

(Artist Rendition) This exoplanet has a blue, Earth-like atmosphere.
Under the red light of its star, it creates a greenish sheen
– Image and description courtesy of Jack H. Madden

Madden created simulated "earths" using combinations of surface types from our own planet, including ocean water, basalt, granite, sand, trees, grass, snow, and clouds. Some of the planets have been simulated with a single surface type, such as jungle / forest planets (like Endor's Forest Moon in Return of the Jedi), snow worlds (like Hoth in Empire Strikes Back), desert worlds … (Tatooine in New Hope), or a combination of several surface types how we find them on earth. Variants of each planet were simulated, including Earth-like ocean cover of 70%, and worlds with and without atmospheric cloud cover of 44%, which correspond to the Earth's average cloud cover. These simulated planets were then placed into orbit the simulated stars in the Habitable Zone – the zone where the planet receives enough energy from the star to sustain liquid water like Earth. These simulated stars had surface temperatures of 3,900 Kelvin to 7,400 Kelvin, which corresponded to 12 different star classes and sub-classes through stars of the F, G and K classes. In this area, stars are cooler and redder than our Sun – Class G and around 5,770 Kelvin – and hotter, bluer stars. Even cooler stars – class M – are excluded, as their planets must orbit so close to their parent stars to support life that they are in danger of being exposed to solar flares and are tide-locked so that they always face the same side of the planet the parent star. A total of 30 different surface types around 12 stars were simulated and resulted in color spectra of 360 terrestrial planets with wavelengths in the range from 0.4 to 20 micrometers (400 to 20,000 nanometers corresponding to the spectrum of visible light to infrared).

“The earth is our only example of a habitable world. The better prepared we are to find something that does not resemble Earth, but is still capable of supporting life, the sooner we can find signs of it … Once we have telescopes that are able to To find signs of life in a distant atmosphere, we have a variety of models to compare against. Based on the observed conditions, we will know what types of surfaces might exist there to support the temperatures for liquid water. "

– Jack H. Madden
(Artist Rendition) This exoplanet and its large moon drift around a bright F star and often create alignments. The heavily scattered incident light hits the clouds
Create a fiery view for observer at the right angle.
– Image and description courtesy of Jack H. Madden

The colors of habitability

The simulated planets form a color reference for future super-telescopes for exoplanet hunting. Comparing the spectra of future exoplanet observations with the simulated Earth-like planets helps determine whether we are seeing a cloudy jungle world, an ocean planet, an airless rock, or a continental world with multiple surfaces like the Earth orbiting various stars. The simulations also showed other interactions between the planet's surface and the incident light from its parent star. Although cooler stars emit less energy than hotter stars, they can heat Earth-like worlds more efficiently, for example, because a larger part of their energy is released as infrared radiation. Depending on how they absorb or reflect light from a particular star, different surfaces also affect the planet's surface temperature. Blue surfaces stay cooler in the blue starlight, while red surfaces absorb more blue light and thus heat. The color contrast of the planet also changes depending on the surface features. A desert planet orbiting a darker K-class star could be twice as bright as an ocean planet orbiting a brighter F-class star, because ocean water is less reflective than sand. Ultimately, the surface type of a planet can have a significant impact on surface temperature and habitability, and can alter the planet's visibility to our telescopes as it orbits a particular star. This information can help in planning which stars we would like to observe with our future super telescopes or which exoplanets we will revisit with our improved resolving power.

Figure 5.2 from Madden 2020, p. 127 – A sample of the combined reflection and emission spectra of the simulated exoplanets with mixed surfaces of 30% of one type and 70% seawater with and without added clouds. (The Y-axis shows the amount of energy reflected from a given surface, while the X-axis shows at what wavelength or color)

The presence of life

The light reflected from the atmosphere of a planet also says something about the atmospheric composition. As the starlight falls through the atmosphere, the light is altered by the presence of various gases that can be detected by telescopes. By simulating all of these worlds, the results get evidence of gases like methane and oxygen in the atmosphere of a distant planet. Typically, methane and oxygen cancel each other out, so their continued coexistence in the atmosphere of a planet, such as on Earth, may be indicative of biological processes in which biology replenishes one or both gases. Vegetation can also be observed at some distance through an effect known as the "red edge" near 700 nanometers – the color red and into the near infrared. At this wavelength, simulated planets covered with trees show a sharp increase in reflectivity. The vegetation on earth reflects infrared light as protection against overheating during photosynthesis. There are some cool options that Madden says are not yet included in the models. For example, it is unclear how a planet's spectrum could change if the planet emits its own light in addition to reflective light. For example, this light could be the result of bioluminescent organisms on the planet's surface (think of the moon Pandora in Avatar). These are possibilities our planet hunt could still discover.

In addition to exoplanet spectroscopy, Madden also models these distant worlds through digital works of art. An example of his work consists of the amazing exoplanet images in this article, including the feature image. I love when the arts and sciences collide. As an astrophotographer, I believe that art is a powerful channel for science communication. Madden's outstanding art can be found on his website – a glimpse into the dreams of a real planet hunter that we can find under the stars:

More to discover

Jack H. Madden's website

The Color of Habitability – Dissertation by Jack Madden

Tour of the Electromagnetic Spectrum (NASA)

Rise of the Super Telescopes: The Giant Magellan Telescope – Universe Today

James Webb works perfectly! On the ground. Next trick: From space – universe today

The search for overinhabitable planets that are even more habitable than Earth – today's universe

The light blue point: Now new and improved – Universe today

Like this:

To like Loading…