A star must contain enough elements for formation of habitable world. Hydrogen and helium are not one of these elements. A star must also last long enough for life to be able evolve. The star should create a steady flow of light for surface life and it should have habitable zones. Lastly the star should be able to create stable and almost circle orbits for worlds. Most stars are capable of being “good suns.” Even around half of all binary stars fit into these criteria.
The first method is called the astrometric method. It is a method that uses very precise measurements of stellar positions in the sky to look for motions, which are caused by orbiting planets. A difficulty with this method is that the motions are supposed to be detected are so small and minor. But it is good for searching planets that are generally bigger and around nearby stars. It is also hard to tell weather steller motion is occurring or not. The next method is the Doppler method. This method searches the spectrum of star’s signs that the star is orbiting a center of mass. This method has been successful but it has three big limitations to it. It is better suited for finding massive planets and it isn’t very effective when it comes to smaller ones. It is easier to study from the ground though but it also requires the right planetary plane and the sun needs to shake as well.
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This method works better if the worlds are close to their sun. This also means that these worlds have shorter orbital periods, which means a shorter amount of time to research them. This method requires a large telescope and long exposure time so that means that there is a small window of opportunity. The third method is the transit method which, transits and eclipses by monitoring a star systems brightness over a long period of time. This method is tedious because it requires observation of the stars brightness for three orbits. It also needs a space observatory and can only view planets edge on orbits.
With todays technology we can figure out a world’s orbital period and its distance form the son. Discovering the planets mass and orbital period to figure out its distance from the sun does this. This also helps us discover its orbital stretch, mass, density, temperature, size, and composition. The size is detected by dips in brightness while it is in transit. Density is figured out by diving the mass by its volume. The temperature and composition can be estimated through transits and eclipses.
The Doppler method can estimate the mass of a world by measuring motions that are caused by a gravitational pull of a planet. The Doppler method can only reveal the motion of a star when it is directed away or towards Earth. Doppler shifts are also only viewable if the spectrum of a star has a planet that has a offset angle and not a head on one. Because of this we are only able to tell a partial velocity, which means we are generally getting the minimum mass.
Worlds that orbit within a habitability zone that have oceans and atmospheres similar to Earth’s have high chances of surface life. If some worlds exit the habitable zone every once and awhile it would be fine as long as they had an atmosphere and oceans that could be used to store heat on the world. It would also have to be a decent size. Any world smaller than Mars can pretty much just be ruled out. The most worlds most likely to be habitable would be Earth sized ones that orbit within the habitable zone. They also can’t be too big because otherwise they could be victims to vertical winds. Moons in the habitable zone that have oceans are promising candidates as well. It’s possible that there could be a greater number of worlds with subsurface habitability than surface habitability. There is a chance some worlds that were once habitable on the surface are now remaining habitable in the subsurface. Some worlds might be able to stay habitable in the subsurface even if they are outside of the habitable zone.
Most stars have planets in their habitable zone. Mainly the Kepler mission discovered this. Because of this I would say that a little over three quarters of stars have planets that are resting in a habitable zone. However there is a fair amount of uncertainty because we are not sure on the characteristics of these host stars.
Kane, and Stephen R. “Habitable Zone Dependence on Stellar Parameter Uncertainties.” [Astro-Ph/0005112] A Determination of the Hubble Constant from Cepheid Distances and a Model of the Local Peculiar Velocity Field, American Physical Society, 14 Jan. 2014, arxiv.org/abs/1401.3349.
Bennett, Jeffrey O., and G. Seth. Shostak. Life in the Universe. Pearson, 2017.
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