Emanating Universe

January 10, 2011

We’re going to focus on a star that’s just within this galactic habitable zone; it’s on the outer edge of the galaxy and is too dim to see from the center of the galactic habitable zone. Our star is called Hemera and it is even punier than our mediocre Sun; while the Sun fuses lighter elements into calcium and gives off a healthy yellow light Hemera is smaller, less compact and looks feverish with its deep orange-red glow. This hue is because Hemera has less mass than our star so gravity doesn’t push down on it as hard, and it’s less dense than the Sun because it burns its fuel more slowly, and with less luminosity. This means that our star will stay in the main sequence stage of its life for longer than the Sun, which is good news for our creatures on this planet orbiting Hemera; it gives them more time to evolve into a form that could one day jump ship before Hemera goes nova.

For our star with a mass of about three-fourths the mass of the Sun, life could potentially live on Areios around Hemera as a main sequence star for about 30.5 billion years before Hemera will go defunct. At 70% of our Sun’s mass, it would only shine at about a quarter of the brightness of our Sun.2 4 This could be problematic for life if the planet isn’t situated close to its star to stay warm. Once a planet gets too close to its star, though, it may become tidally-locked, like our moon is to the Earth, with the same side perennially pointing towards the surface of Hemera. One side of the world would boil and the other side would freeze. A planet can overcome this with an atmosphere thick enough to circulate heat and keep one side of the planet’s atmosphere from boiling and the other side from freezing.7 Curiously, the planet would be divided into three distinct zones; one of perpetual light, one of perpetual darkness, and one of perpetual twilight between the two opposing hemispheres of light and dark. 6 Astrobiologist Nancy Kiang suspects that plants on this world would be jet black to absorb as much of the dim sunlight as possible around a red dwarf star.8

Habitable Zones

Note that the Sun-like Star on the right is the most moassive and has the widest habitable zone of the three stars.

For a planet orbiting a red dwarf star, the only way to keep warm is to orbit in this tidally-locked configuration that would keep one side of the planet always pointing to the star and one side always pointing away, but a new model of planet habitability suggests that planets orbiting a red dwarf may be host to habitable planets. Red dwarf stars can exist in the main sequence stage for tens of billions or maybe even a 100 billion years, certainly longer than the age of the universe so far.5 This would mean that a red dwarf star could be habitable to life for tens of billions of years, much longer than on Earth or Areios. Also, because these stars don’t burn out at the rate of more massive stars, they make up a greater percentage of the stars in the night sky. Astronomers estimate as many as 75% percent of the stars in the universe could be red dwarf stars.10 This makes planets within the habitability zone of red dwarves a priority for astronomers looking for habitable planets, if only technology were sensitive enough to detect such dim stars and their planets. However, there are an abundance of them for planet hunters to find.

It should be mentioned that a star will get brighter with age. Over the lifetime of a star, the luminosity increases as mass decreases and gravity pushes down on the star with more intensity. As a star loses fuel, gravity pushes it harder towards a center point which concentrates the remaining fuel it has left into a more efficient sphere that burns harder to correct for the now overpowering gravity to bring the star back to equilibrium. When stars are just forming, they release high-energy radiation that would sterilize the surface of any planet too close; this activity lessens with time, but it would leave the surface of a planet uninhabitable at first. Low-mass red dwarf stars undergo these shifts in luminosity called flares to a much greater extent than a star like Hemera.9 Once Hemera gets over this early phase, it will produce less ultraviolet radiation than our sun, and generate more infrared and visible light, making it potentially safer for life on the surface. Our Sun has grown about 33% brighter over the last 4.5 billion years.3 At first Areios may be less habitable for life because it’s so cold orbiting around a dim star, but over time that star would get hotter and brighter.

The surface temperature of a planet isn’t solely dependant on the amount of radiation it receives from its star, either. Surface temperature also depends on how effectively the atmosphere can trap heat in a phenomena called the greenhouse effect. Without the greenhouse effect, Earth would be much cooler and while carbon dioxide levels have fluctuated over geologic time, there is ubiquitous consent that rising carbon dioxide levels are causing a more pronounced greenhouse effect. The greenhouse effect occurs when certain greenhouse gases like water vapor, carbon dioxide, and methane allow shorter wavelength light to pass through the atmosphere, but traps longer wavelength infrared light, to keep it from escaping.1 Infrared light heats up the planet enough on Earth to keep the tropics from freezing, but when humans pumped more carbon dioxide into the atmosphere from burning fossil fuels, it trapped more heat and caused a series of climate changes that we have yet to figure out what the long-term impact of that decision will truly be. The same mechanism that intervenes to maximize the surface temperature on our planet wouldn’t be as pronounced on a world like Areios with a hyperactive plate tectonic system to scrub carbon dioxide from the planet. The greenhouse effect that is causing widespread climate change on Earth could keep Areios warm enough to form liquid water on the surface.

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