Seeing the planets for the trees

May 20, 2011 By Anuradha K. Herath
Two photographs of a black spruce forest in Canada - the left one taken with the Sun behind the observer (back-scattering), and the right one with the Sun opposite the observer (forward-scattering). Credit: Don Deering

A recent study says that a particular mathematical technique could be used to detect forests on extrasolar planets.

In the search for , scientists are looking beyond and are developing techniques that would help them detect multicellular . In a recent study published in the journal , researchers are proposing a particular to detect tree-like multicellular structures on .

“This technique allows us to identify planets that potentially have complex life and distinguish them from planets with simple life,” said lead author Christopher Doughty, a junior research fellow in tropical science at the Environmental Change Institute at the University of Oxford in England.

In other words, the authors predict that even when observing planets outside the solar system, scientists would be able to identify a planet with forests by the characteristics of the light that it reflects, even if it looks like just a dot in the viewing lens.

Removing the Shadows

Scientists have come up with various methods to detect life on extrasolar planets. Some are working to detect the composition of gases in the atmosphere -- the presence of oxygen, which on Earth is mainly a by-product of life, could be one such biosignature. Others are focusing on a reflectance signature such as the “red edge,” which is the difference between the light that is absorbed for photosynthesis and the light reflected back in other wavelengths of the near-infrared spectrum.

The diagrams show some of the different causes of BRDF. Credit: Wolfgang Lucht

The diagrams show some of the different causes of BRDF. Image Credit: Wolfgang Lucht
When studying climate on Earth, it is important to understand the brightness of its surface. In order to determine the brightness, scientists must first account for the effect of shadows that are cast by various structures on the surface. The mathematical technique used to estimate the effect of shadows is what is known as bidirectional reflectance distribution function, or BRDF. It is defined as the change in reflectance of an object viewed from different angles. The method is already used on satellites orbiting Earth.

“Imagine going outside on a sunny day,” Doughty explained. “When the Sun is directly overhead, you will not see your shadow. If someone took a picture of you from above, there would be no shadows present, and the picture would be bright. Now go outside a few hours later. Your shadow will be present. A picture taken from above will now be darker because the shadow is included.”

Trees on Earth developed their canopy form due to the competition for light and the need to transport water and nutrients. The authors of the study predict that if multicellular photosynthetic organisms are found on extrasolar planets, they too will be found to have a tree-like structure that casts shadows.

Doughty and his co-author Adam Wolf of Princeton University used a BRDF model to simulate vegetation and how it reflects light at different planetary angles to estimate how the brightness of the planet changed with and without trees. The authors contend that, when viewed from space, forests appear brightest when the observer is in line with the Sun.  This position is known as the “hot spot” because at that location, no shadows are visible.

“Even if the entire planet were reduced to a single pixel, under certain situations, there would be a difference in the brightness of the planet as it rotates around its star that would not be there if there were no trees,” Doughty said.

There are other factors that affect a planet's brightness, such as the presence of water. Light reflecting from a planet that is covered in water or ice will make that planet look much brighter than a planet without those features.

The Hunt for Life

With the discovery of more and more planets outside the solar system, there is growing interest in devising methods to detect life on Earth-sized extrasolar planets. To do this, scientists rely on planetary biosignatures or indications that serve as evidence for both life that may have existed in the past and may exist now.

In addition to atmospheric biogenic gases such as oxygen, another biosignature scientists have already considered is the surface reflectance spectra of vegetation, or the amount of light reflected off plant matter at different wavelengths.

If plants are found on other planets, they could look very different compared to those found on Earth. Some astrobiologists think plants on other planets will have a different dominant color than green. Credit: Tim Pyle / Caltech

“There are also in situ techniques to look for chemical signs of life in the rock surface of a planet, but this requires actually visiting the planet for direct sampling, like the Martian Rovers are doing,” said Nancy Kiang, a scientist specializing in terrestrial biometeorology and biogeochemistry at NASA’s Goddard Institute for Space Studies in New York City.

"Astronomical techniques with telescopes look at the radiance spectrum of a distant planet. Up till now, the target biosignatures for telescopes have been biogenic gases and surface biological pigments."

The technique proposed in this study, however, is different.

“This technique [BRDF] can distinguish between a planet dominated by single cellular life and that of tree-like multicellular life,” Doughty said.

Kiang said the study was a “good start with an original contribution to the concept of biosignatures.” She pointed out that using the BRDF function would require knowing another biosignature – the reflectance spectrum of a plant leaf – in order to select the suitable wavelength to calculate the function.

“This could help distinguish complex life from, say, green slime,” Kiang said.

Explore further: Young binary star system may form planets with weird and wild orbits

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User comments : 13

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2.4 / 5 (5) May 20, 2011
Life evolved here to adsorb the spectrum of light emitted by the parent star - the Sun.

The sun emits and chlorophyl adsorbs visible light.

Forests elsewhere may adsorb other wavelengths.

With kind regards,
Oliver K. Manuel
Former NASA Principal
Investigator for Apollo

1 / 5 (3) May 20, 2011
So we are looking for shadows?
I'm sure I saw a tree on the edge of a crater on the moon.
2.8 / 5 (5) May 20, 2011

You realize "Adsorb" is the wrong word, right?

"Adsorb" is when something collects material on it's surface layer.

Absorb is when something assimilates incident radiation into itself.

The wavelengths that a life form could aBsorb is detemined by it's chemistry, so a hypothetical photosynthetic alien life form could only exist for an off-colored star if there is sufficient trace element chemistry with appropriate aBsorbtion spectra to make use of the incident radiation.

I don't know what the point is anyway.

There is no reason to believe life exists on any other planet, and even if it does, there is no reason to believe it would be in any way similar to life on earth, even if it is carbon based life.

There is no reason to believe it would have DNA or RNA.

There is no reason to believe it would have genetic material.

There isn't even reason to believe it would be "cellular" life, because we can imagine totally different types of organization.
not rated yet May 20, 2011
We already know of many types of theoretical molecular machines which are already being designed and modeled in the cutting edge nano-technology research.

Once you make the assumption that life exists elsewhere, then you need to consider ever possible form of molecular machinergy that could be organized into systems of "organelles".

There could be life "somewhere" which is quite differnt from our cellular life.

The systems could be in symbiotic relationships which are not "self contained" or may be on the surface of carbon compound rods or sheets, rather than contained inside membranes.

This would have an disadvantage of being more vulnerable to the environment, but it might have an advantage in genetic diversity and materials transport mechanisms.

Additionally, the molecular machinery at the "organelle" level may be so radically different that the protection of a membrane is no longer important.
not rated yet May 20, 2011
Consider our intracellular matrix have things like bone, cartilage, tendons, etc, which the cells make to form complex multi-cellular life.

A system in which the organelles are not contained may build tissue and organs by building proteins or other structures directly on the spot, instead of by stacking cell membranes, cell walls, or intracellular matrixes to make tissues.

I'm thinking more like an assembly line, rather than our cellular bags of organelles which grow until they split...
5 / 5 (4) May 20, 2011
I think what the article is saying is that they have a technique to search for life, that would be more complementary than primary.

Essentially they're saying that the specific topography of plants/trees throws shadows in such a way as to alter the brightness of the landscape in distinct ways during the planet's rotation, whereas water/liquid causes distinct reflections, and a primarily rock/earth/soil landscape throws shadows in a different way.

i don't think it was explained well in the article, and it was kinda lost among the explanation of all the other techniques.

That said, i think it has potential.
1 / 5 (9) May 20, 2011
You know how powerful and how good of resolution a telescope would need to be to detect the shadow of a "Tree" on an exoplanet?

Hubble cannot even make out anything smaller than continent sized surface features on Pluto, which is only like 4 billion miles away.

Gliese is 118 TRILLION miles away, which is 29,500 times farther away.

So you'd need something about 30,000 times better than Hubble to take images of similar resolution at Gliese as Hubble does at Pluto.

Then you'd still need to get at least 3 or 4 orders of magnitude better in order to test for shadows of "trees" or "forests".

Excluding revolutionary breakthroughs, it costs about 10 times as much in order to make a telescope or microscope 1 decimal place better. So the cost of making a telescope or array capable of detecting a "tree" line on Gliese, if one exists, would be hundreds of thousands to millions of times more than Hubble...

You could send a probe for less than the build cost of the telescope.
1 / 5 (1) May 21, 2011
Whoa there, Mr Spectator. Do you know what it would "cost" past, now dead, societies to communicate "effortlessly" across thousands, nay millions of miles? More than the totality of all their fortunes combined. Because "technology" is not simply a quantifiable price of some amount of money. Monetary value is a fiction, which needs to be adjusted on a relative basis in order to allow society to progress. Today, we are told by the powers that be that we must live within our means and balance the budget. The fact is that we have squandered our future to an insane ideology of "too big to fail" in order to keep these powers that be fat and happy, while they tell the rest of their "subjects" to tighten their belts. What is needed is the political will to send these speculators into a much deserved hell with their monetarist worship of money delusions. The House of Representative bill HR 1489 seeks to restore Glass Steagall. Let us stop being mere spectators on this financial Titanic.
5 / 5 (3) May 21, 2011
@spectator - This isn't an issue of resolution. It's regarding the cumulative effect. For example, if the planet has water, it will brighten signifacantly when it orbits/rotates at the right angle for its sun to reflect off the water.

With plants, as it orbits there will be a point where the topography of the biosphere will create a distinct variability in amount and type of light reflected at any moment that is distinct from a lifeless planet.

Think of it this way. remember the 'blackest black' thing that was created using a forest of carbon nanopillars to help absorb more light? And normal black paint if it is very smooth can still be reflectice and shiny? Sometimes You can tell the difference between something that is smooth or microscopically rough by whether it is matte or glossy. (try sandpapering your glossy car)

The principles aren't all exactly the same, but it's a similar analogue to what they are looking for here.
1 / 5 (4) May 21, 2011
That guy:

See, there's a fallacy here.

Oceans don't reflect much light.

The water itself doesn't absorb light very well, but the dissolved material and suspended debris does.

Ocean water only reflects about 8% of sun light.

Wet soil and forest are similar at about 8%.

This means you would not be able to tell the difference between an ocean, a swamp or marsh, or a forest based on albedo or shadow effects caused by changes in angle of incidence of radiation.

not rated yet May 22, 2011
Basic assumption must be changed: looking for known/Earth-life-type effects should be secondary as doing so would search for life similar to ours while all data suggests we might be just one of many possible pathways for life.
To see all possible life-forms we need to focus on looking for unusual. Unusual anything and everything. Looking for differences in extrasolar planets. From there, look where data points and where the special character of that ecosystem is.
2 / 5 (3) May 22, 2011

You realize "Adsorb" is the wrong word, right?


not rated yet May 24, 2011
That guy:

See, there's a fallacy here.

Oceans don't reflect much light.

The water itself doesn't absorb light very well, but the dissolved material and suspended debris does.

Ocean water only reflects about 8% of sun light.

Wet soil and forest are similar at about 8%.

This means you would not be able to tell the difference between an ocean, a swamp or marsh, or a forest based on albedo or shadow effects caused by changes in angle of incidence of radiation.

Water reflects on average 8% of the sun, however as anyone knows - if you're at the wrong angle to the sun on a sunny day, for very narrow angles, the reflection is much much higher.

Look, I'm not even saying this method they're proposing is practicle - it's interesting, but I'm not sure it will work in the real world - but what I am saying is that the concept behind it is valid.