I wrote this as an extended definition or explanation of some scientific concept. Specifically, I explain some of the methods we use to detect extra-solar planets.
Searching for a Needle in the Cosmos
We have long searched for a planet that is as hospitable to life as the Earth. Such a planet would have to have a solid surface and be large enough to maintain its orbit, but not too large as to crush any life from gravitational influence. The planet would also have to be at the correct distance from its host star as to stay at a reasonable temperature and it would have to have a sufficient amount of atmosphere made out of non-toxic gases. We have evaluated the ability of the planets within our solar system to be hospitable to life. Some have potential, but most fall short of our requirements. This is not a problem for astronomers, though, because there are billions and billions of planets hundreds of light years away, happily orbiting a star that is not our sun. Any one of them could have the traits needed to host life. Not wanting to leave these extra-solar planets uninvestigated, astronomers have developed techniques to detect planets outside our solar system. The new techniques not only detect planets, but also inform us about the presence of characteristics necessary for supporting life on said planets. How can this be? How are astronomers able to detect these pinpoints of rock and gas in our ever-expanding universe, these needles in a cosmological haystack? And how can they determine if these planets can host life?
One of the most common methods astronomers use to detect and measure the mass of extra-solar planets is the transit method, in which the brightness of the planet’s host star is consistently measured. The transit method only works for star-planet pairs that are oriented in such a way that the planet will pass in front of and obscure part of the star as seen from Earth. This “edge-on” view is in contrast with an orientation where the planet looks like it is tracing out a circle around the star. Astronomers will record and graph the brightness of the star over some amount of time. Typically, the graph will show some variation in brightness.
Many factors may account for the variability of a star’s brightness. The star may be producing stellar spots, which are equivalent to sun spots, dark patches of low temperatures in comparison to the rest of the star. The star may be rotating or it may occasionally be obscured with dust. The way astronomers deduce what is responsible for the variability in the brightness is by looking for a particular pattern in the changes.
What sort of pattern should we expect to see if a planet is causing the changes in brightness? Try imagining a planet off to the side of a star. We would measure the full, regular brightness of the star. This level of brightness would continue until the planet started to transit, or pass, in front of the star. Then the detected brightness of the star would continually decrease until the entire planet overlapped the star. The measured brightness would stay at a minimum value until the leading edge of the planet reached the edge of the star. At that point, the brightness level would gradually increase back to its original, maximum value, as more of the star was revealed as the planet moved past it. As the planet continued its orbit, it would eventually pass behind the star. We would then measure a small decrease in brightness since the light reflected by the planet from the star would no longer reach Earth. Then, as the planet came around the star again, the whole process would repeat itself. So a plot tracking the brightness of a star would indicate the presence of a planet if it showed a large dip, then an increase back to a maximum value, then a small dip, then an increase back to the maximum value again, and the pattern repeated.
The transit method for detecting extra-solar planets is useful for helping astronomers figure out how large a planet is, which, when combined with information gathered from other sources, can help them infer what the planet is made of and how strong gravity is there. From there, the astronomers can hypothesize about whether such a planet would be hospitable to life. The ratio of the height of the large drop to the small drop gives astronomers the ratio of the surface areas of the star and the planet. If the radius of the star is already known, then astronomers can calculate the radius of the planet. By considering the densities of planets with similar radii, astronomers can then estimate the density, and therefore the mass, of the planet in question. Knowing how large a planet is can then tell them if it is more likely to be gaseous or solid. Currently, astrobiologists are more interested in solid planets, because they are more likely to be similar to Earth, and so more likely to be hospitable to life.
We need to know more than how large a planet is before we can conclude if it would be hospitable to life. Life, at least as we know it, can only survive within a certain range of temperatures. To estimate the temperature range of a planet, astronomers must know how large the host star is and how far away from the star the planet is. The “radial velocities” method of detecting extra-solar planets, in which the gravitational effect of the planet on the star is measured, helps inform astronomers about the distance between a detected planet and the star it orbits. Methods used to calculate the mass of a star are a subject for a different time.
We often think of a planet orbiting the center of a star, but, in reality, the star and the planet are orbiting a common point in space. Consider a see-saw. If there are two five-year olds of equal mass playing on the see-saw, then they can sit equally far away from the center to remain balanced. If we replace one of the five-year olds with a football player, who is presumably heavier than the five-year old, then the football player must sit closer to the center of the see-saw to maintain balance. The center of the see-saw happens to be the where the “center of mass” of the five-year old and the football player is located. Now imagine the see-saw was taken away, but the five-year old and the football player remained suspended in the air. The center of mass of the two is still where it was when the see-saw was there. If we moved the football player or the boy, the center of mass would also move. The center of mass of an object, or a group of objects, is a point in space at which all the mass in the system is balanced. The center of mass may not necessarily be made up of physical matter.
Star and planet systems work pretty much the same way balancing a see-saw does, only instead of moving up and down while the center of mass stays in place, the star and planet orbit the center of mass while it stays in place. A large difference between the mass of the planet and the mass of the star means that the center of mass will be closer to the star. As the planet gets more massive, the center of mass moves away from the star. This does not mean that the more massive planets are closer to the star; just that they have a greater effect on the location of the point that the star and the planet orbit around than smaller planets. For example, Mercury is closer to the sun than Jupiter, but Jupiter is far more massive. Because the difference in mass between the sun and Jupiter is smaller than the difference between the sun and Mercury, Jupiter affects the sun’s motion more than Mercury does. A larger distance between the center of mass and a star makes the difference between the star revolving in place and the star actually moving in a small elliptical orbit.
How do astronomers take advantage of the fact that a planet can affect a star’s motion by shifting the center of mass away from the star? If a planet is large enough to cause a star to move in a noticeable orbit around the center of mass, the star is sometimes moving away from Earth and sometimes moving toward Earth (assuming, once again, that we have an edge-on perspective of the system). As the star moves away from the Earth, the radiation emitted by the star has a longer wavelength, and as the star is moving toward the Earth, the radiation is compressed and has a shorter wavelength. Astronomers measure the changes in the wavelength of the radiation emitted by the star. If the changes in wavelength are large, astronomers know that the star must be traveling in a bigger orbit. After calculating how large the orbit of the star is based on measurements of the changes in the wavelength, astronomers can estimate how large a planet would need to be in order to have an effect of the calculated size on the orbit of the star, assuming they have used other methods to find the mass of the star.
Once the astronomers know how large a planet is, they can then find how far away the planet is from the star through Newton’s law of universal gravitation. For every planet-star pair there is something called the “Goldilocks zone”, the range of distances from the star in which the planet would be at a temperature hospitable to life. Astronomers and astrobiologists are hoping that their calculations will lead them to find a small (Earth-sized), rocky planet within its Goldilocks zone, as this planet would have a higher probability of being hospitable to life than other kinds of planets.
The main constraint on these methods is that it is easier to find large planets, planets a few times the size of Jupiter. Most of these tend to be gaseous, which isn’t quite what we are looking for. However, some rocky planets with the mass of several Earths have been discovered. These discoveries should motivate improvement of the instruments used to uncover extra-solar planets. Currently, astronomers can detect changes in velocity as small as one meter per second! That is about as much as Jupiter perturbs the sun’s orbit. Though it may take years of research, it is within our abilities to create instruments able to detect Earth sized extra-solar planets. Once we can do that, we may eventually discover a planet as welcoming to life as Earth. Such a discovery would be as incredible as finding a diamond encrusted needle glinting amidst a pile of dust and hay.
Author’s Note: In the course of writing this paper, astronomers published results saying they had detected planets Earth-sized and smaller through the Kepler mission.
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