Assignment: Stellar Evolution
Based on what we know about life on Earth, we think life requires three things: energy, nutrients (CHONPS), and liquid water. There are plenty of energy sources in the universe (sunshine, chemical sources, geothermal sources, radioactivity, tidal heating, etc.), and there are enough heavy elements to seed Earth-like planets. We think liquid water is the limiting factor.
We define the stellar habitable zone as the orbital region surrounding a star where a terrestrial planet might have just the right temperature to sustain liquid water, between 273 K and 373 K. We can calculate the range of orbital radii that define the stellar habitable zone using the following equation.
r=(77,470)(T2L0.5)
In this equation, the luminosity of the star (L) is in units of solar luminosities, the desired planet temperature (T) is in units of kelvin (do not confuse this with the star's temperature!!!), and the orbital radius (r) at which that temperature is likely is in astronomical units. The constant out in front takes into account unit conversions so you don't need to make those calculations yourself.
I. Use your known stellar luminosity data to calculate the inner and outer edges of the habitable zone for each of the star systems in our data set. Record this information in the table provided.
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Inner Edge (AU)
Where T = 373 K
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Outer Edge (AU)
Where T = 273 K
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Haedus
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Vega
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Sun
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Ran
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II. Based on your results for the Sun, which planets in our solar system are in its stellar habitable zone. Do the results surprise you? Explain.
This stellar habitable zone calculation assumes that a planet absorbs 100% of the light that strikes it. This isn't always the case. Earth, for example, has an albedo of 0.3, reflecting 30% of the sunlight striking it back into space. That means it only absorbs 70% of the sunlight that hits it.
III. What sorts of things do you think would increase a planet's albedo?
IV. What do you expect to happen to a planet's average temperature if it doesn't absorb 100% of the starlight that strikes it? How do you expect this would change your habitable zone estimates?
This stellar habitable zone calculation assumes that a planet has no atmosphere of any kind.
V. How might having an atmosphere affect a planet's average temperature? If we included atmospheric effects in our calculation, how do you expect that would change your habitable zone estimates?
Exoplanets
Since 1995, we have discovered about 5,000 extrasolar planets, in other words, planets outside of our solar system.
In an exciting discovery late last year, astronomers found two exoplanets orbiting Ran. They've since been named Eminiar and Vendikar. Both are classified as terrestrial planets, roughly the sizes of Mars and Earth respectively. Their orbits are much more compact than those in our solar system. Eminiar is 0.15 AU from Ran while Vendikar is 0.27 AU from the star.
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Mass
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Orbital Radius
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Eminiar
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0.15 Earth Masses
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0.15 AU
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Vendikar
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0.8 Earth Masses
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0.27 AU
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In about 5 billion years, our Sun will become a red giant. In rapid succession it will engulf Mercury and Venus, fry Earth to a crisp, then die out as a puny white dwarf. No planet in our solar system will ever be habitable again. Assuming humanity is still around in 5 billion years, we will need to immediately relocate our population to nearby star systems, and the Ran system is a promising target.
VI. Which planet in that system is most likely to be habitable for us? Why?
VII. What sorts of engineering projects would we need to complete to make our chosen home more hospitable?
VIII. Ran is a very different star than our Sun. How might its stellar properties pose a challenge to our existence?
The nebular hypothesis of star formation predicts that planet formation is a natural consequence of star formation. As a cloud contracts it heats up and spins faster; the faster rotation causes the cloud to fling material out into a protoplanetary disk. If the nebular hypothesis is correct, all star systems should have planets, yet none are currently known to exist in the Haedus system. The most likely explanation is that no one has bothered to look for planets orbiting Haedus.
IX. Why might an astrobiologist ignore stars like Haedus in the search for habitable worlds?