Simple Colors Could Provide First Details of Alien Worlds

At best, the few extrasolar planets we have imaged directly are just points of light. But what can that light tell us about the planet? Maybe more than we thought. As you probably know the, Deep Impact spacecraft flew by comet Hartley 2 today, taking images from only 700 km away. But maneuvering to meet up with the comet is not the only job this spacecraft has been doing. The EPOXI mission also looked for ways to characterize extrasolar planets and the team made a discovery that should help identify distinctive information about extrasolar planets. How did they do it? By using the Deep Impact spacecraft to look at the planets in our very own solar system.

The spacecraft imaged the planetary bodies in our solar system — in particular the Earth, Mars and our Moon — (see here for movies of the Moon transiting Earth) and astronomer Lucy McFadden and UCLA graduate Carolyn Crow compared the reflected red, blue, and green light and grouped the planets according to the similarities they saw. The planets fall into very distinct regions on this plot, where the vertical direction indicates the relative amount of blue light, and the horizontal direction the relative amount of red light.

This suggests that when we do have the technology to gather light from individual exoplanets, astronomers could use color information to identify Earth-like worlds. “Eventually, as telescopes get bigger, there will be the light-gathering power to look at the colors of planets around other stars,” McFadden says. “Their colors will tell us which ones to study in more detail.”

On the plot, the planets cluster into groups based on similarities in the wavelengths of sunlight that their surfaces and atmospheres reflect. The gas giants Jupiter and Saturn huddle in one corner, Uranus and Neptune in a different one. The rocky inner planets Mars, Venus, and Mercury cluster off in their own corner of “color space.”

But Earth really stands out, and its uniqueness comes from two factors. One is the scattering of blue light by the atmosphere, called Rayleigh scattering, after the English scientist who discovered it. The second reason Earth stands out in color is because it does not absorb a lot of infrared light. That’s because our atmosphere is low in infrared-absorbing gases like methane and ammonia, compared to the gas giant planets Jupiter and Saturn.

“It is Earth’s atmosphere that dominates the colors of Earth,” Crow says. “It’s the scattering of light in the ultraviolet and the absence of absorption in the infrared.”

So, this filtering approach could provide a preliminary look at exoplanet surfaces and atmospheres, giving us an inkling of whether the planet is rocky or a gas planet, or what kind of atmosphere it has.

EPOXI is a combination of the names for the two extended mission components for the Deep Impact spacecraft: the first part of the acronym comes from EPOCh, (Extrasolar Planet Observations and Characterization) and the flyby of comet Hartley 2 is called the Deep Impact eXtended Investigation (DIXI).

14 Replies to “Simple Colors Could Provide First Details of Alien Worlds”

  1. What a brilliant and simple idea. I look forward to seeing plots like this for dozens of extrasolar planets; with lots of points some dramatic relationships should become apparent.

  2. @NEXUS: I agree. I eagerly await larger and better resolution scopes that would allow this method of exoplanet plotting.

  3. This is very good news.

    With this technique, we would have to adjust for red-shifting, and star type correct?

    What are the chances next generation telescopes could image Earth-analogs in in a useful way? Seems like getting any images of small rocky worlds like ours would require means beyond even next-generation scopes… or am I wrong?

  4. Excellent idea but hard to achieve. There is no telescope large enough to get the information of color of extrasolar planets so far. Wish next generation large ground based telescopes or space telescopes could take it.

  5. The blue signature is a signal for oxygen. Oxygen has strong Rayleigh scattering of light in the 3-400 nm range of blue light.


  6. Oh, and of course measurements beat modeling. I’m just trying to sanity check – and there’s something airy about the results.

  7. Early days but very exciting. On a totally different topic have any of the non-American subscribers here had difficulty understanding the presenter’s accent and enunciation? I had to play the clip twice to catch every word. As has been said, the British and Americans are two people separated by a common language.

  8. I thought I recognised this idea; i.e. Characterizing Extra-Solar Planets with Color Differential Astrometry on SPICA; Abe, L. (2009)
    This Japan / ESA mission (with collaborations with Canada, USA and South Korea. It plans to do Color Differential Astrometry (CDA).

    As they say for SPICA (Sp)ace (I)nfrared Telescope for (C)osmology and (A)strophysics;

    “The nature of these extrasolar planets will be revealed by mid-infrared spectroscopy of planetary atmosphere also by SPICA.”

    NASA looks like it is trying to preempt this project in light of its more and more desperate measures for more limiting funds from the US Government,. Shows nothing is new under the sun!!

  9. Obviously, spectroscopy is nothing new. But I think this video is directed more at people who don’t know what a spectrum is, in the physics sense of the word. Maybe, as pointed out above, this piece has the funding in mind and hence they have to make it Very. Simple. To. Understand. since they want to reach the politicians. Maybe… But anyway, it’s more like something for a science class at secondary school then a thorough scientific presentation.


  10. @ Hydrazine

    Oh dear. Clearly you have no scientific education (or you missed that lesson.)

    I talk about spectrophotometery and you think it is basic spectroscopy!! There is an significant difference between these terms – mostly.because individual colours are measured by a photometer with calibrated filters, while spectroscopy uses a prism or grating to obtain the whole spectrum (many colours).

  11. @ Hon. Salacious B. Crumb

    Yeah, yeah, yeah… nitpicking. Spectrophotometry is a special application of spectroscopy. Go look it up…

  12. “Spectrophotometry is a special application of spectroscopy”

    Absolutely wrong. No it is not. I’m a colourist, so I should know the difference!!

    Taking you foot out of you mouth, then putting the other one in, just shows you are foolish!

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