Star Light, Star Bright…Any Oxygen Tonight?

Star Light, Star Bright, Any Oxygen Tonight?

Click here for larger image. The Sun, a typical G2V dwarf. G stars are characterized by the presence of metallic lines and weak hydrogen.
Credit: Harvard University

Breathe deeply, and thank the nearest tree, bush, or blade of grass. The Earth receives most of its oxygen from photosynthetic activity.

Before the development of photosynthesis and the subsequent rise of oxygen, life on Earth remained single-celled. Oxygen is largely seen as necessary for the development of multi-cellular complex life – in other words, higher plants and animals (including humans).

The question of complex life on other worlds, therefore, is tied to the possibility of photosynthesis development. But could photosynthesis develop on worlds orbiting stars different from our sun?

Look at the night sky, and you may notice that stars of different colors blaze out from the darkness. The color of a main sequence, or "dwarf" star is determined by its temperature: the hotter O, B, and A stars are shades of blue. The cooler K and M stars run from orange to red. Mid-temperature F and G stars are white or yellow. Our yellow sun, for instance, is a G star.

Despite the principal color of a star, the visible light it emits contains the full rainbow of the color spectrum. Photosynthetic plants on Earth rely on such visible light from our sun to produce food and oxygen. The photosynthetic pigment chlorophyll absorbs every color wavelength to varying degrees.

The most efficient photosynthetic wavelengths are blue wavelengths measuring about 450 nanometers. Photons, or packets of light energy, contain different amounts of energy based on their wavelengths, and shorter wavelengths contain more energy than longer wavelengths. Thus, the shorter blue wavelengths pack more of an energetic punch.

According to Ray Wolstencroft of the Royal Observatory in Edinburgh, Scotland, and John Raven of the University of Dundee, Scotland, plants prefer blue light not only for the efficiency of the wavelengths, but also because blue light played an integral role in the origin of photosynthesis.

The red colors of Orion.
Credit: A. Vannini, G. Li Causi, A. Ricciardi and A. Garatti

Before photosynthesis developed, the early Earth had little oxygen. That meant the Earth also did not have an ozone (O3) layer to protect life from the damaging ultraviolet (UV) radiation of the sun. In lieu of an ozone layer, early life on Earth lived underwater, relying on the ocean for UV protection. However, the ocean also filtered out the longer wavelengths of visible light. Thus, the only wavelengths available for the earliest photosynthetic pigments were the shorter wavelengths of blue light.

Because blue light is preferred by photosynthetic life on Earth, it would seem that planets orbiting hot blue stars would have the greatest amount of photosynthesis.

But the hotter the star, the more UV radiation it emits. On Earth, one of the main impacts of UV exposure on plants is a reduction in the rate of photosynthesis. According to Wolstencroft and Raven, the amount of UV radiation produced by a star is a major limiting factor to photosynthesis development.

So while the hot blue stars emit a lot of the blue visible light that photosynthesis on Earth prefers, the higher UV output of these stars makes the development of photosynthesis less likely.

The shorter lifetimes of such hot stars also act as a barrier to photosynthesis development. Hot blue stars burn out quickly, in 10 or 20 million years. Compare that to G stars like our sun, which have a lifetime of about 10 billion years (the sun is middle-aged, being about 4.7 billion years old).

On Earth, photosynthesis probably didn’t appear until our planet had been around for at least a billion years. Due to other planetary processes that determine how much oxygen stays in the atmosphere, photosynthesis didn’t produce atmospheric oxygen approaching modern levels until about 500 million years ago. If the evolution of photosynthesis is the same everywhere, then the lifetime of blue stars is just not long enough.

Blue stars in the Pleiades. These stars produce more UV radiation than red stars.
Credit: DSS and LTImage

(In addition, O and B stars tend to stay near the stellar birthplace, so planets might have a difficult time forming and surviving amid the intense gravitational interference of such star clusters.)

On the opposite side of the spectrum, the cooler M and K stars emit much less UV radiation. They also have very long lifetimes – M stars, for instance, are thought to last for a hundred billion years or longer (well beyond the present age of the universe!). These factors would work in favor of photosynthesis development.

However, these stars emit mostly longer wavelength radiation approaching the infrared. Photosynthesis on Earth requires eight photons of visible light to produce one molecule of oxygen, whereas twelve to sixteen photons of light beyond the visible range would be needed to carry out the same task in a modified version of photosynthesis. Such a high light requirement might result in reduced rates of photosynthesis on terrestrial planets orbiting a cool red star.

Of course, the development of photosynthesis on Earth did not just rely on the sun’s wavelengths. It also was dependent on various factors specific to our planet, such as cloud cover, atmospheric composition, amount of land, and the depth of the ocean. Since so many factors played a role in the evolution of photosynthesis on Earth, perhaps photosynthesis could evolve on other worlds orbiting less-than-ideal stars.

In addition, other planetary factors perhaps could speed up the rate of photosynthesis development. There does not seem to be a particular reason why photosynthesis must develop at the same pace as it did on Earth.

Targeting the Zone

GL623, a red dwarf
Credit: NASA

As long as planets have enough liquid water oceans and cloud cover to protect against UV radiation, say Wolstencroft and Raven, planets orbiting most stars should be able to develop photosynthesis. One key to finding such planets is to look in the star’s habitable zone.

The habitable zone – or the orbital region around a star where water can remain in a liquid state – is seen as the most promising target in the search for life in the universe. Habitable zones for hotter stars will be further away than the habitable zone in our own solar system (which lies roughly between the orbits of Venus and Mars).

Red stars, meanwhile, are so much cooler than our sun that their habitable zones would have to be much closer. This could result in a tidally locked orbit where one side of the planet is always facing the star, much like our moon does with the Earth. Unless such a planet had an atmosphere capable of transferring heat effectively, one side of the planet would be fiery hot while the other would remain ice cold.

In a search for planets around stars within 30 light years, the most common stars to be found are cool red dwarfs. Yet planet hunters mainly confine themselves to searching for extrasolar planets orbiting F and G stars similar to the sun, and sometimes K dwarf stars. The habitable zone around such stars should be similar to our own.

Wolstencroft and Raven say that the greatest photosynthetic generation of oxygen will be on Earth-like planets orbiting F dwarf stars. But while F stars might be more photosynthetically productive than our sun, because they are hotter they have a much shorter life span.

"F main sequence stars have a lifetime of 3.2 billion years, compared to our sun’s 10 billion-year life span," says Wolstencroft. "While these stars may produce the most oxygen, they have a much shorter window of opportunity."

So how would we find out if a distant planet has photosynthetic life? One way would be to find an oxygen (O2) signal as part of a planet’s light spectrum. Oxygen is a highly reactive gas and cannot remain in the atmosphere in substantial quantities unless something is continually producing it. Since most of the oxygen on Earth is produced by photosynthetic organisms, the existence of oxygen on other worlds would be strong evidence for photosynthetic life.

Unfortunately, oxygen has a weak infrared spectral signature, and would be very difficult for us to detect with present technology. But ozone (O3) has a very strong signal. Because ozone is produced by the interaction of UV light and oxygen, planets with an O3 signal will also have large amounts of O2 in the atmosphere.

Atmospheric water vapor over Earth’s Western Hemisphere. Water vapor is a key factor that points to life on a planet.
Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio

But not everyone agrees that an ozone signal would be proof of photosynthesis. James Kasting, a professor of geoscience at Pennsylvania State University, says there are at least two circumstances where substantial amounts of O2 and O3 might be produced abiotically.

"The first is a runaway greenhouse planet like Venus," says Kasting. "The second is a frozen, Mars-like planet, outside the habitable zone. [This type of planet would have to be] slightly bigger than Mars so that it could hold onto its oxygen."

However, Kasting says that we could distinguish between abiotic and biotic sources of oxygen with additional observational data. For example, a planet orbiting within the star’s habitable zone would suggest a biotic source for the oxygen. Another factor pointing to life would be if the planet’s spectrum showed gases like water vapor and carbon dioxide in the atmosphere.

Kasting believes that photosynthesis is equally likely on any terrestrial planet orbiting a star in its habitable zone. As far as Kasting is concerned, the type of star does not matter as much as the elements available on a planet. He says the only elements that are necessary for the development of photosynthesis are liquid water and carbon dioxide, and he believes they probably are available almost everywhere.

"Photosynthesis on Earth is limited by nutrient availability – nitrogen, phosphorus, and iron – not by photons, except at very high latitudes," says Kasting. "I suspect the same would be true on planets around other stars."

What’s Next

NASA’s Terrestrial Planet Finder will allow scientists to directly detect spectral signatures from extrasolar planets. By analyzing the colors of infrared radiation, astronomers will be able to search for carbon dioxide, water vapor, and ozone. The Terrestrial Planet Finder is scheduled for launch sometime between 2012 and 2015.

The European Space Agency’s Darwin telescope also shows promise for detecting the spectral signatures of life. Due to launch sometime around 2015, this telescope will survey thousands of star-planet systems looking for ozone, carbon dioxide, water vapor, and methane.

Related Web Pages

Color and Light
Chemistry and Spectroscopy of Planetary Atmospheres
What is Photosynthesis?