Revolving around the sun, in an orbit similar to our own, is NASA’s Kepler Spacecraft. Launched in 2009 and named after the Renaissance computer science homework help astronomer Johannes Kepler, Kepler’s mission, like that of many ground-based telescopes, is to survey a portion of the Milky Way and discover exoplanets – planets outside of our solar system. Ultimately we hope to find a habitable world. The impetus to find such a world echoes a familiar predicament that recently came to light in the Pixar film Wall-E. What if Earth one day becomes uninhabitable and humans must move to a new home? Ideally, we would rather find a planet like our own than spend our days in a free-floating spaceship. Colonization aside, simply knowing if life exists elsewhere would answer one of humanity’s most fascinating questions: what else is out there?
As of July 13 2013, nearly 1000 exoplanets have been identified, a quarter of which may be habitable [1,2,4]. The number of planet candidates, or planets that have been detected by one method but not yet been confirmed by another, rank in the thousands, and each month, the Kepler telescope alone spots hundreds more. Scientists estimate that each star in our galaxy has at least one orbiting planet. This means that our galaxy is likely home to at least 100 billion exoplanets and potentially, 17 billion earth-sized planets, sometimes referred to as twin Earth or Earth 2.0 [1,2,3].
How do we find exoplanets?
If you peer through a telescope onto a clear night sky, you may be able to spot the rings of Saturn. In terms of distance, Saturn is 1.5 light hours away, meaning light from Saturn’s rings takes 1.5 hours to reach us. The nearest exoplanet candidate, Alpha Centauri Bb, is 4.4 light years away , meaning light from that planet takes 4.4 years to reach us. As result, less than 5% of exoplanets can be seen with telescopes.
How, then, have we been able to find the rest of the exoplanets? While not used by Kepler, most exoplanets have been found using a technique called radial velocity, or Doppler spectroscopy. The principle behind this technique is that we can detect tiny changes in a star’s radial velocity caused by an orbiting planet. A star’s radial velocity is the speed it moves towards or away from an observer. The gravitational pull of an orbiting planet tugs on the parent star and causes the star to ‘wobble’ or move around their common center of gravity. As the planet moves around the star, the observer sees a shift in the star’s radial velocity. This shift in radial velocity is dependent on the size of the planet and its distance from the star. A larger planet that is closer to its parent star will cause the radial velocity to shift more. Jupiter, for instance, shifts our sun by 12 m/s while the earth shifts it by less than 0.1m/s because Earth is much smaller than Jupiter [3,5]. Below, you can see how a planet’s movement around a star causes it to appear to be moving toward or away from an observer.
Image by Reyk via Wikimedia Commons.
Current Doppler spectroscopy instruments are only precise to ~1m/s. This means scientists cannot use this method to discover smaller planets that shift their parent star’s radial velocity by less than 1 m/s, and this method biases scientists to finding larger planets. The up-side of Doppler spectroscopy is that it does not require more extensive equipment than a well-tuned spectrometer, an instrument that detects light emissions (hence no need for huge telescopes or even space telescopes). The reason for this simplicity is we are not actually directly observing the star but looking for shifts in the star’s light emission that could suggest a wobble. The down-side of Doppler spectroscopy is that it requires time, approximately 500 to 1000 observations per wobbly star  and can only be used with stars whose wobble causes them to move closer and further away from us as opposed to side to side shown here.
Image by Reyk via Wikimedia Commons.
The other dominant exoplanet detection method is the transit method. As a planet passes between its parent star and an observer, the star’s observed brightness dims. This dip in stellar brightness is what space telescopes like Kepler pick up. Jupiter-sized planets can block up to 1% of the star’s light whereas Earth-sized planets may only block 0.01%. Detection of smaller planets, like Earth, is heavily dependent upon precision and timing. The method as a whole has been likened to spotting a seagull fly across a lighthouse beam . Below is a figure adapted from Science magazine that shows a planet moving across a star and the resulting variations in stellar brightness and radial velocity (Stellar RV) . In panel B, note the dip in stellar brightness and the ‘wobble’ on the radial velocity curve as the planet passes across.
Figure 1. Illustration of a planetary orbit and its effect on stellar brightness and RV. (A) The planet orbits the parent star counterclockwise and eclipses the star. (B) The observer detects the planet by either the dip in the stellar brightness (transit method) or the wobble in the stellar RV (Doppler spectroscopy). Both methods are necessary to confirm a planet’s existence. From A. W. Howard, “Observed Properties of Extrasolar Planets”, Science 340, 572 (2013). Reprinted with permission from AAAS.
Doppler spectroscopy and the transit method are complementary. The transit method frequently produces false positives (‘planet’ signals that are due to something else) and relies on ground-based methods like Doppler spectroscopy to confirm a planet’s existence. Moreover, while planet size and orbit can be inferred from transit data, mass, most often, cannot. Mass, however, can be measured from follow up spectroscopy observations . Together, the two methods provide the bulk of a planet’s physical attributes—mass and radius. Density can then be calculated by dividing the planet’s mass by its volume.
Once scientists know mass, radius, and density, they can make educated guesses about an exoplanet’s atmosphere and core composition. For example, an exoplanet with a low density (~0.5 g/cm3 compared to Earth’s 5.5 g/cm3 density) but a relatively large mass (8 times as heavy as Earth) may suggest a substantial amount of lighter gasses (like hydrogen and helium) in the atmosphere. An exoplanet with a high density (~9 g/cm3) and a relatively small mass (4 times as heavy as Earth) is more likely to have a rock/iron core with little to no atmosphere . Exoplanets with densities close to that of water may be ‘water worlds’ covered in oceans or sheets of ice .
Direct imaging is needed to truly determine the makeup of a planet. This approach gathers light from the planet itself and disperses it into a spectrum . Coronagraphs (telescopic attachments designed to block light) are used to block out light from the parent star. The planet’s emissions of infrared light are then measured, as planets are usually brighter in infrared than they are in visible light. Nevertheless, gathering planet light is not always easy. The parent star is millions to billions of times brighter than the planet and plucking a planet out from its star’s glare is sometimes impossible. This is much easier if it is a large planet in a wide orbit around a dim star. Exoplanets like this are usually large and gas-filled, and are what we find most often.
While astronomers watch intently for exoplanets showing up as small blips in stellar radial velocity or stellar brightness, the general public envisions exoplanets as the artist renders them, orbs aglow in the finest stardust. But what are some of these worlds really like? In our search for the habitable, we have come across worlds that are truly alien. There are pulsar planets that orbit fast spinning pulsar stars left behind after a supernova. These stars are very dense and emit pulses of electromagnetic radiation. Planets orbiting pulsar stars are trapped by the star’s strong gravitational field and become very dense themselves; one such planet is said to be made of pure diamond. There are rogue planets that orbit no star and float aimlessly through space, homeless and exceptionally cold. There are also circumbinary planets that orbit binary stars or two star systems and thus see two suns. And, in stark contrast to the water worlds mentioned above, there are also volcanic worlds, planets so tethered to their parent stars that they see nothing but 4000°F heat and rain of molten rock .
Exoplanets also come in an assortment of sizes, from the very large Jupiter-size planets, to the modest, Earth-size ones. Jupiter-size planets make up the majority of confirmed exoplanets. Puffy planets are hot, Jupiter-size planets that are so light they could float on water if given a large enough tub. One size down from the Jupiters are the Neptunes and the mini-Neptunes. One size down from the Neptunes are the Super-Earths and the Earth-size planets . Below is a table that illustrates the current common mass classification of exoplanets in terms of size with respect to Earth. An exoplanet with enough pull, usually a Jupiter sized one, may even have an exomoon spinning faithfully around it.
How close are we to ET?
Although we have discovered many different types of planets, we have not yet found extraterrestrial life. Nonetheless, current instruments are probing for the likes of ET in the habitable zones of planet-sustaining stars. The “habitable zone” is the region around a star where a planet can have the right temperature to maintain liquid water [7,8]. The zone changes with the type of planet and type of star; a larger, brighter star, for instance, has a wider habitable zone and a drier, rockier planet may only be habitable at the inner edge of the zone. It is important to note that just because a planet is in the habitable zone does not mean it is habitable [8,10]. Venus, Mars, and Earth all fall within our sun’s habitable zone; however, Venus is too hot and Mars is too cold but Earth, as Goldilocks might say, is just right. To search for life, scientists look for exoplanet emissions of biosignature gases that can be detected by spectrometers . Biosignature gases are produced by life on the planet (microbes, plants, people, etc) and gather in the atmosphere at high levels. Today’s telescopes do not have the capacity to detect these gases but future telescopes may. Hence the race to find life is also a race to advance science.
A conventional habitable planet is covered in liquid water. Water is the crux of life as we know it but perhaps this is not the case for every planet, perhaps somewhere in the ether is a gas planet teeming with new life. Given the number of exoplanets in our galaxy, other possibilities may very well exist. As stated in a recent Science article, ‘planet habitability is planet specific’  and no universal rule can apply.
Weike Wang is a graduate student at the Harvard School of Public Health
 J. Schneider The Extrasolar Planets Encyclopedia http://exoplanet.eu/catalog.php.
 Homepage for Kepler Telescope http://kepler.nasa.gov/
 Exoplanets, Worlds Beyond our Solar System (space.com) http://www.space.com/17738-exoplanets.html
 M. Cruz, R. Coontz, “Alien Worlds Galore”, Science 340, 565 (2013).
 Y. Bhattacharjee, D. Clery “A Gallery of Planet Hunters”, Science 340, 566 (2013).
 L. Wade, “A Glossary of Their Quarry”, Science 340, 570 (2013).
 A. W. Howard, “Observed Properties of Extrasolar Planets”, Science 340, 572 (2013).
 S. Seager, “Exoplanet Habitability” Science 340, 577 (2013).
 Strangest Alien Planets (space.com) http://www.space.com/159-strangest-alien-planets.html
 Hot on Trail of ‘Just Right’ far-off Planet (New York Times) http://www.nytimes.com/2011/12/03/science/space/scientists-are-hot-on-trail-of-exoplanets-suitable-for-life.html