Now that we have explored Earth, Venus and Mars in some detail, it is very tempting to conclude that Venus is too close to the Sun so it is too hot for life as we know it. Mars is too far away from the Sun so it is too cold, and, as the Goldilocks fairytale goes, Earth is just the right distance from our Sun, neither too hot nor too cold to support life. This is the essence of the concept of the Goldilocks Zone, a specific distance from a star in which a planet receives just enough energy to support liquid water on its surface, and thus, life.
However, if we think about these three planets for a moment, we might notice some complications to this idea. First of all, of these planets, only Earth has a significant magnetic field that protects its atmosphere from solar wind. Why? It is this atmosphere that moderates Earth’s temperatures and allows liquid water to exist, between 0°C and 100°C. On Mars, with no magnetic field, any atmosphere that may have once existed has been stripped away over the eons by solar winds. Its surface is now ravaged by ultraviolet and other solar radiation. Earth is also the only planet that exhibits plate tectonics and this means that nutrients and gases can cycle into different forms, keeping their atmospheric concentrations buffered within certain extremes. For example, carbon dioxide is moderated by the carbon cycle on Earth and this prevents a runaway greenhouse effect, which seems to have occurred on Venus, sending its surface temperatures skyrocketing up to about 460°C.
Does distance from the Sun have anything to do with internally created core dynamos or plate tectonic movement? Does how a planet form impact its later habitability? Do catastrophic events early on in a planet’s life affect its later habitability as much or more than its distance from its star? For example, did Earth’s surface water come from within Earth or did it come from comet impacts during the Heavy Late Bombardment? Let’s see what the Goldilocks Zone theory has to offer in terms of answering these questions.
What the Goldilocks Zone Is
The Goldilocks Zone or habitable zone is, technically, the intersection of two regions in space that must both be favourable to life. One region is confined to the planetary system of interest and the other region is where this system exists within its galaxy. The zone around a star, also called the circumstellar habitable zone or ecosphere, is where the star’s energy output allows for water to exist as a liquid rather than freezing or boiling away. The galactic habitable zone is a hypothesis that is met with a bit more skepticism than the ecosphere zone. The idea here is that the center of the universe acts in much the same way as a star does in the ecosphere zone. The favourable zone must exist close enough to gather enough heavy elements to form a habitable planet but far enough away to be protected from radiation from the galactic centre. Planets or moons could exist within this intersected region and possibly support carbon-based life. However, it is important to remember that planets within the Goldilocks Zone may not all be habitable. For example, gas giants in this zone are unlikely to support life.
The Goldilocks Planets Of Gliese 581
The red dwarf star, Gliese 581, is of particular interest to scientists studying Goldilocks planets because it has a planetary system and one planet in particular, that lie within the Goldilocks Zone. This image produced and copyright by the European Southern Observatory (Gliese 581’s planetary system is described in detail on this Wikipedia page) compares Earth’s and Gliese’s Goldilocks planets.
(copyright attributed to European Southern Observatory)
You can see that both Earth and Mars are within the Goldilocks Zone with Venus just on the inner edge of it. Gliese 581 g exists well within the habitable zone around its star, with Gliese 581 c and Gliese 581 d straddling the zone. Scientists currently hold all three exoplanets as possibly life-supporting, with reservations. Gliese 581 g appears to be a perfect candidate for alien life. However, recent evidence indicates that it may be tidally locked, meaning that one side always faces its star. And I should also note that both planets g and f are listed as unconfirmed because they have not been detected by new spectrograph analysis. In both solar systems, planets such as Gliese 581 f and Jupiter do not receive enough solar radiation to make up for radiative losses, and surface water freezes. Planets such as Gliese 581 e and Mercury, on the other hand, absorb too much solar radiation and any surface water simply boils away. Keep in mind that the Goldilocks Zone must be calculated for each star system based on the energy output of that star. The Goldilocks Zone for Gliese 581 is much closer to the star than our Sun’s habitable zone. Gliese 581 has only 0.2% of the visual luminosity of our Sun (but while our Sun radiates mostly in the visible spectrum, Gliese 581 radiates mostly in the near infrared, meaning that although it is much fainter than the Sun it gives off a much higher percentage of its radiation as heat than the Sun does). This NASA image gives you an idea of how the Goldilocks Zone (shown as a green belt) is affected by the host star’s energy output.
Kepler Space Mission Results
The Kepler Space Observatory launched by NASA in 2009 is designed to discover Earth-like planets orbiting within the habitable zones of other stars in the Milky Way galaxy. This is Kepler’s targeted star field, only 1/400th of the night sky, courtesy of NASA.
We do not yet have the capacity to directly observe these planets but we can detect them indirectly by monitoring fluctuations in brightness of other main sequence stars. Fluctuations in brightness indicate one or more planets and/or moons crossing the star’s surface. This NASA image shows how this works.
In February 2011, the Kepler Space Observatory Mission Team released a list of 1235 possible exoplanets, with 54 of them being both Earthlike in size and existing within the Goldilocks Zone. This 10 - minute NASA video introduces us to the Kepler Mission.
This is a very exciting time for astronomy. These results have allowed scientists to estimate for the first time that about 6% of all stars host Earth-size planets and 19% of all stars host multiple planets. Astronomer Seth Shostak believes that, based on the Kepler findings, there are at least 30,000 habitable worlds within a thousand light years of Earth!
New Missions To Study Goldilocks Planets?
We need to confirm the existence of the Kepler Mission exoplanets and to study them in more detail and for that we will need a new generation of observational missions tailored to study exoplanets. Several such missions have been proposed and await funding. The Darwin, a European Space Agency cornerstone mission, was proposed but scrapped in 2007, because of technical and funding problems. It would have detected exoplanets and then carried out more detailed analysis of their atmospheres, looking specifically for the presence of oxygen. Finding just atomic oxygen does not necessarily mean life, however. Europa, one of Jupiter’s moons, has a tenuous oxygen atmosphere that is produced by the radiolysis of water molecules (this means that solar radiation breaks water molecules into ionized atoms) high up in its atmosphere. To find oxygen produced biologically through some kind of photosynthesis-like process, astronomers must look for the simultaneous presence of ozone, water and carbon dioxide. Oxygen produced at high altitude as it is on Europa immediately attacks atmospheric ozone and prevents its accumulation. If oxygen is produced low in the atmosphere, say through photosynthesis, and little water gets high into the atmosphere, then there are no ions that can attack ozone. Therefore, scientists now believe that ozone, water and carbon dioxide (required for photosynthesis) along with oxygen comprise a reliable biosignature in an alien atmosphere. This mission would have looked for this signature, with the Gliese planets being good first candidates for study.
Other missions that have been proposed and scrapped or put on hold include the New Worlds Mission, PLATO, the Space Interferometry Mission, the Terrestrial Planet Finder and the Transiting Exoplanet Survey Satellite, all of which were planned at least in part by NASA except for PLATO, which was another planned European Space Agency mission. Right now, the Kepler Mission, still active, is our best exoplanet explorer and it has been very successful so far. As our technology improves, new missions to study these worlds will undoubtedly be planned and we will move forward in this exciting new field of research.
Goldilocks Zone – A Concept In Infancy
In the meantime, it is important that we refine our search for habitable planets. The Goldilocks Zone concept is in its infancy and it will be refined and expanded upon as we learn more about what makes planets habitable. We are doing that right now as we study the mysteries of Mars, Venus and Earth right here in our own stellar neighbourhood. We also need to question and refine our definition of life itself. As we discover new forms of life never before believed possible in extreme environments right here on Earth, where there is no oxygen or temperatures are well below freezing are well above the boiling point, we must open ourselves to new possible extreme habitats in which life finds a foothold. And we must explore the possibilities of biochemistries that may not be based on carbon or water at all. Why is liquid water so important for life to exist? Carbon compounds dissolve in water to form the basis of all life as we know it, from enzyme functions to the building of cells, tissues and more complex structures. But are other biological solvents possible? A potential biosolvent should exist as a liquid over the range of temperatures an alien organism might encounter, and pressure must be accounted for. For example, while hydrogen cyanide has a very narrow temperature range as a liquid at 1 atmosphere (the surface pressure on Earth), it can exist as a liquid over a wide temperature range on Venus where the surface pressure is almost 100 times greater. Observations from NASA’s Spitzer telescope hint that planets around cool stars such as M-dwarfs and brown dwarfs, which are widespread around the Milky Way, might offer a prebiotic chemical soup that is different from that of our young Earth. The disc chemistry around these cool stars is different from that around our Sun, containing significant amounts of hydrogen cyanide, for example. Hydrogen cyanide is an active molecule that can combine to form adenosine, an essential building block of DNA. Perhaps it could function as a building block in the biochemistry of some kind of alien life, perhaps on a young planet such as the one in this NASA artist’s conception.
Methane, hydrogen fluoride and perhaps even molten salts could also theoretically be used as biosolvents. As well, rather than carbon, life could use silicon atoms. Silicon is chemically similar to carbon and it is far more abundant on rocky planets than carbon is. However it cannot create as many diverse functional groups as carbon can and it can’t readily form the double and triple bonds important to carbon-based biochemistry. But it may be useful under temperatures and pressures different than those of Earth or it could be used in roles less analogous to carbon. These kinds of studies will affect what we refer to as the Goldilocks Zone and perhaps even do away with the concept all together.
Here are some intriguing articles to explore:
Life As We Didn’t Know It – An ecosystem that thrives in complete darkness
A New Form Of Life – A new extremophile is discovered in California’s exotic Mono Lake
‘Goldilocks Zone' Bigger Than Once Thought - To find worlds within the "Goldilocks" zone, where conditions to support life are just right, look no further than our own solar system
Cruising the Goldilocks Zone - The Search for "Earth's Twins" - this article outlines a comprehensive shopping list for the ingredients to make a habitable exoplanet
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