An extrasolar planet-which I will
call an exoplanet from here on-is a planet that exists outside of the solar
system. For years, exoplanets were
merely hypothetical, with astronomers dating back to Newton and beyond,
postulating on their existence; but there was no proof that they existed. The search for exoplanets promises not
only extending scientific knowledge of the universe, but also, with a more
existential flavor, the answer to that mighty question: are we alone in the
universe? With hundreds of
billions of stars in just the Milky Way, not even considering the other
billions of galaxies in the universe, there are an estimated 100-400 billion
exoplanets in the galaxy. In 1988,
the first exoplanet was discovered using a radial velocity technique. The planet, Gamma Cephei Ab, was not
confirmed until 2002, however it can claim the honor of being the first exoplanet
discovered. Gamma Cephei Ab is
around 45 light years from Earth-making it relatively close compared to most
exoplanets. This planet is between
2 and 16 times more massive than Jupiter.
Since the discovery of Gamma Cephei Ab in 1988, scientists have
confirmed 919 planets outside of the solar system. So far, the most clearly observed and evidenced exoplanets
divide into three types: gas giants, hot super-Earths, and ice giants. The hunt is on for a fourth type of
exoplanet: terrestrial planets-that is a planet one half to 2 times as massive
as Earth and located within the habitable zone, which we will discuss further
later on in the blog post.
Scientists use different
techniques to detect and confirm an exoplanet. Such techniques include but are not limited to: doppler
shift, astrometric measurement, transit, direct imaging, and gravitational
microlensing. Now we won't go into
these techniques in detail, for want of time and a more qualified instructor,
but it is useful and enlightening to keep in mind that multiple methods and
disciplines are incorporated into the search for exoplanets. One of the most famous and successful
missions in the search is the now decommissioned Kepler Space Telescope, with
which astronomers were able to confirm 156 planets, and identify another 3602
candidates. Kepler uses planetary
transits to detect exoplanets. As
we all know, a transit occurs when a planet crosses in front of its star. When a planet transits its star, it
causes a small dip in the star's brightness-a change of about 100 parts per
million. This tiny change is
detectable by Kepler, and since orbits are periodic, astronomers can observe the
star over time and see if the planet really is a planet. Now that the planet is detected,
astronomers can use Kepler's Third Law to calculate the period and mass of the
planet. They can then in turn use
this information to calculate the size and temperature of the planet. Thus far, the count for confirmed exoplanets goes as follows: Stars with Planets 712; Multi-planet Systems 145;
Gas Giant 407; Hot Jupiter 432; Super Earth 70; Terrestrial 13; Unknown 9. The 13 terrestrial planets all lie within the habitable zone. The
habitable zone, sometimes referred to as the "goldilocks zone" is the
distance from a star in which a planet could potentially harbor life. Now we're getting into some interesting
stuff; the continuing discovery of more and more planets in the habitable zone
only strengthens the argument that we are not alone in the universe. Only a few days ago, on November 5, a
NASA report (using Kepler data) estimated that at least one in five stars in the Milky Way have at least one planet that is Earth-sized and located within
the habitable zone. If you feel
your mind being blown, that's okay, it is only natural. This means that if you look up at the
stars at night, the nearest star with the possibility of a planet is only 4
light years away and is visible; you could see a star with terrestrial planets
and potentially life. It is
discoveries like this that drive the hunt for exoplanets. The human thirst for answers, a thirst
that led to the discovery of fire, a thirst that led to the discovery of the
Americas, a thirst that led to the discovery of manned flight, will in time
lead to the discovery of life in the universe.
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