All living things have one thing in common. All living things are carbon-based and that they need water to survive. Water is therefore important to sustain life. There is no need to elaborate the reason why water is a crucial ingredient in a habitable planet. Water must be used as a primary requirement in the search for Earth-like planets.
Scientists pointed out that water remains in liquid form if the planet is in a distance that would enable it to “receive sufficient radiation from the star to maintain the effective surface temperature of the planet above the melting point of water, 273 K” (Shaw 204). This is an important statement because water can appear in three basic forms, liquid, gas, and solid.
It is imperative that water remains in a liquid state in order for life to exist. However, if one considers the environmental conditions outside planet Earth, one will find that water cannot remain liquid beyond what is called the habitable zone.
The habitable zone or HZ is the optimum condition and “this condition defines a habitable zone around a star and it is the range of distances from which an orbiting planet will have liquid water on its surface” (Shaw 204).
In the case of the Earth, “the habitable zone is determined by two factors: the effective surface temperature of a planet as determined by the flux arriving form the local star and the radiation-trapping efficiency atmosphere around the planet” (Shaw 204).
This is an interesting definition because it also takes into consideration the necessity to trap heat because of the presence of an atmosphere. The HZ is an important topic of discussion when it comes to the search for life outside planet Earth.
Extrasolar Planet Search
The search for life outside planet Earth must begin with the study of extrasolar planets. For many centuries, learned men were convinced that the Earth is not unique and that there are Earth-like planets that are orbiting stars like the Sun (Casoli & Encrenaz 2). By using this concept, scientists made the bold declaration that as of today, there are at least 200 extrasolar planets that have been discovered by NASA (Casoli & Encrenaz 2).
However, it must be clarified that these extrasolar planet are not really similar to Earth (Casoli & Encrenaz 2). As of this point it is safe to say that no extrasolar planet has yet been discovered that closely resembles the Earth in terms of habitable zone and the presence of liquid water.
The desperate search for life in outer-space can force others to become overly zealous about their pronouncements. There are also those who would like to interpret these statements based on their own biases. Such was the case of Dimitar Sasselov, a member of the Kepler mission team tasked to search for extrasolar planets (Moskowitz 1).
Sasselov was quoted to have said that his team were able to discover 100 Earth-like planets. But upon closer examination, Sasselov said that the Kepler mission discovered possible “candidates” and these are yet to be confirmed as extrasolar planets.
When a planet orbits around its parent star, it momentarily blocks the light from the said star and when viewed from a telescope the “dimming” effect is unmistakable. However, it requires more extensive study to determine if it is indeed an extrasolar planet that causes the partial blocking of light. It is also imperative to find out if these extrasolar planets are similar to Earth in terms of HZ and the presence of water.
There are at least five major methodologies that can be used to detect extrasolar planets and these are listed as follows: 1) Doppler Shift; 2) Astrometric Measurement; 3) Transit Method; 4) Gravitational Microlensing; and 5) Direct Detection. The Doppler Shift is based on the Doppler Effect.
According to scientists “the Doppler Effect is the apparent change in the wavelength of radiation caused by the motion of the source” (Seeds & Backman 101). In application of this principle, NASA scientists were able to discover that “precise measurement of the velocity or change of position of stars tells us the extent of the star’s movement induced by a planet’s gravitational tug” (PlanetQuest 1).
It is a clever way of determining if there is a planet that orbits a particular star. However, there can be other factors that are creating the Doppler Shift. But more importantly there is no way to clarify if this planet resembles the Earth in terms of size and if it orbits within a habitable zone.
The Doppler Shift technique can easily detect giant-sized planets orbiting close to a star such as those with masses similar to Jupiter. However, the main goal of the Kepler Mission is not only to detect extrasolar planets but Earth-like planets. Therefore, there is a need to refine this technique.
The Astrometric Measurement methodology provides this capability. When astronomers in the Kepler Mission “use astrometry, they look for a minute but regular wobble in a star’s position” (The Planetary Society 1). This methodology, however, requires sophisticated telescopes that ideally must be launched into space to overcome the effect of the interference of the Earth’s atmosphere.
Although the Astrometric Measurement is a refined technique in the sense that it can detect even the slight wobble of the star in relation to its interaction with an orbiting planet, it has one major limitation.
The main weakness is that it cannot help astronomers determine the exact size of the extrasolar planet. Nevertheless, the Astrometric Measurement enables astronomers to increase the number of extrasolar planets that they can detect and analyse.
It is therefore imperative to have another tool at their disposal. The Transit Method is based on the idea that as a planet orbits a star it blocks the line of sight between this particular star and an observer located on planet Earth. It is also based on the principle that there is a regular pattern to the way planets orbit around a star and it leads to the following methodology:
If astronomers detect what seems to be a transit, they will continue to monitory the star, seeking to find the next transit… if a third transit occurs after the same time interval, counting from the second transit, as the interval that separates the first and second transits, we have excellent confirmation that the method has indeed found a planet (Goldsmith & Owen 412).
A similar strategy that utilises the behaviour of light from a star in connection to a planet orbiting around it is the Gravitational Microlensing. The idea came from Einstein’s theory of general relativity and it is based on the principle that “light rays bend when passing through space that is warped by the presence of a massive object such as a star” (PlanetQuest 1).
Therefore, as a planet passes the line of sight of an observer, the light that travels from its parent star is distorted by gravity. As a consequence, the lens-effect makes the light appear brighter to an astronomer (PlanetQuest 1).
The Transit Method must be used to complement the Doppler Shift method, Astrometric Measurement method and Gravitational Microlensing. These three methods must be used as preliminary strategies and confirmation can be achieved with the use of the Transit Method.
Telescopes and Satellites
The Transit Method can confirm the existence of an extrasolar planet orbiting a star that is similar to the Sun. However, the ultimate goal of the astronomers is to locate an extrasolar planet that shares similar characteristics with planet Earth. Thus, it is of utmost importance to develop tools that would enable astronomers to use Direct Detection methods.
The other previous methods are based on mathematical computations that cannot confirm the existence of a rocky planet with an atmosphere. A Direct Detection method erases all doubts and would confirm not only the existence of an extrasolar planet but also the fact that this particular planet can support life.
At the most basic level Direct Detection methods rely on sophisticated telescopes that use cutting-edge technology. For example, the Keck telescope in Hawaii is the largest in the world (The Planetary Society 1). Telescopes that are on Earth are severely limited by interference. However, possible solutions are described below:
Two space missions, now in their planning stages, will make possible measurements of unprecedented accuracy within a few years. NASA’s Space Interferometry Mission (SIM) has been delayed multiple times, and is now scheduled to launch no earlier than 2015.
Once in orbit around the Sun, SIM will be able to make angle measurements of single stars as accurate as 1 micro arcsecond – the width of a human hair at a distance of 500 miles (The Planetary Society 1).
The SIM project seems expensive and is a showcase of what NASA can offer. However, if one takes a closer look at the information provided, it is clear that these space-bound telescopes are not yet designed to focus on planets but stars.
In other words, it is extremely difficult to look for extrasolar planets because of its distance from Earth. An Earth-like planet is small compared to the star that it orbits. Searching for it is like looking for a needle in a haystack.
Most Promising Extrasolar Planets
An objective assessment of the Kepler Mission and other endeavours geared towards the discovery of extrasolar planets can be simplified through the following statement “So far, astronomers have only turned up huge planets that probably don’t harbour life” (PlanetQuest 1). However, astronomers are optimistic that someday they will be able to discover an extrasolar planet that is located in an HZ similar to the solar system.
The announcement of the discovery of Gliese 581g, an extrasolar planet that was supposed to be located in the “Goldilocks zone” created excitement in the scientific community.
But it turned out to be an unsubstantiated claim. A Geneva Observatory astronomer named Francesco Pepe made the declaration that his team could not find this particular planet (Vergano 1). In fact, the Kepler Mission did not announce any discovery related to this extrasolar planet.
It is interesting to point out that the Kepler Mission boldly declared that they have found 1235 extrasolar planets. However, there is a clarification at the end of that statement, saying that these are just candidates and requires confirmation. A closer look at the data will reveal that as of the present there are only 24 confirmed planets.
Consider for instance the seven confirmed extrasolar planets on their list and it is hard to believe that they program will succeed. Take for example the following candidates:
- Kepler 1b;
- Kepler 2b;
- Kepler 3b.
The first three planets on the list are gigantic planets, and more often than not, bigger than Jupiter (Ames Research Centre 1). Kepler 5b, the fifth planet on the list is twice the size of Jupiter (Ames Research Centre 1). Another problematic issue is that these planets require a very short time to orbit around their parent stars therefore putting serious doubts when it comes to their location in the habitable zone.
Detailed Study and Potential Communication
The 24 confirmed planets require detailed study. The ultimate step is to have visual confirmation of these planets and go beyond mathematical computations such as tracking star movements and the behaviour of light coming from a particular star. Astronomers must develop powerful telescopes that can be launched into space and observe the planets unimpeded by atmospheric interference.
For astronomers who would like to think far ahead, a plan must be established in order for astronomers to communicate with inhabitants of other planets. A contingency plan of this nature must be ready in case the Kepler Mission succeeds. The first hurdle that has to be overcome is how to travel from Earth to a distant planet.
According to one commentary a spaceship the size of a yacht, travelling half the speed of light will require millions of pounds of fuel to reach a planet located four light years away. On top of that, the whole trip will take eight years to complete (Seeds 325). There is also the linguistic challenge (Scharf 10). Scientists must work with linguists and other experts to learn how to communicate with aliens.
As of this point there is no evidence that there is life outside planet Earth. It has proven to be extremely difficult to find an Earth-like planet in terms of size, atmosphere and location in a habitable zone. But astronomers are not going to give up because of the sheer size of the universe.
They believe that they are going to discover an Earth-like planet someday. Planet detection methodologies must be refined to increase the efficiency of astronomers searching for extrasolar planets.
Ames Research Centre. Kepler Discoveries. NASA, 6 Nov. 2011. Web.
Casoli, Fabienne and Therese Encrenaz. The New Worlds: Extrasolar Planets. New York: Springer, 2005. Print.
Goldsmith, Donald and Tobias Owen. The Search for Life in the Universe. CA: University Science Books, 2002. Print.
Moskowitz, Clara. Claims of 100 Earth-Like Planets Not True. Space.com, 22 July 2010. Web.<https://www.space.com/8808-claims-100-earth-planets-true.html>.
Scharf, Caleb. Extrasolar Planets and Astrobiology. CA: University Science Books, 2009. Print.
Shaw, Andrew. Astrochemistry: From Astronomy to Astrobiology. UK: John Wiley & Sons, Ltd., 2006. Print.
Seeds, Michael. Perspective on Astronomy. CA: Thomson Higher Education, 2008. Print.
Seeds, Michael and Dana Backman. Astronomy: The Solar System and Beyond. CA: Cengage Learning, 2010. Print.
The Planetary Source. Astrometry: The Past and Future of Planet Hunting. The Planetary Society, May 2009. Web. <http://www.planetary.org/explore/>.
Vergano, Dan. Goldilocks Planets Waiting for Just-Right Rockets. USA Today, 18 Oct. 2010. Web. <https://usatoday30.usatoday.com/tech/science/columnist/vergano/2010-10-15-interstellar-travel_N.htm>.