SUMMARY: an extrasolar planet, or exoplanet, is a planet outside of our solar device that orbits a star other than ours Sun.

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Sections:Detection MethodsGeneral properties of well-known ExoplanetsComparing Exoplanets to Planets in ours Solar SystemKepler MissionRelated LinksRelated Lessons:Space take trip Guide

Swiss astronomers Michael Mayor and also Didier Queloz discovered the first exoplanet in 1995. Because then, the variety of verified exoplanets has surpassed 200. There are a couple of different methods scientists use when trying to detect extrasolar planets.

Detection Methods:Direct Evidence:Images the the planetSpectra of the planetIndirect Evidence:Doppler techniqueTransits

Direct evidence of exoplanets is very complicated to obtain. Think about the photos we have of Pluto together taken native Earth. The finest pictures we have present Pluto as a an extremely small, round blob. It"s difficult to take it a great quality snapshot of something that is so far away. Pluto orbits, ~ above average, around 40 time farther from the sunlight than Earth. That"s nice far. Now think around a earth that is 4 million times farther indigenous our sunlight than us are. You have the right to imagine how challenging it is for united state to acquire a picture of miscellaneous that much away. Not just are they really much away, yet planets space really dim contrasted to their parental stars for this reason they"re really tough to see. Indirect observations (such together the Doppler technique, transits, and also eclipses) room much an ext commonly offered when in search of exoplanets.

The Doppler result on a star(click to enlarge)

The Doppler an approach is a great method for discovering exoplanets. It supplies the Doppler effect to analyze the motion and also properties the the star and planet. Both the planet and also the star are orbiting a common facility of mass. This method that the star and also the world gravitationally attract one another, bring about them come orbit approximately a allude of mass main to both bodies. The is favor trying to perfect balance a large child and also a little child top top a teeter-totter. If the teeter-totter is exposed in the center, the larger child will certainly be ~ above the ground, however if you move the larger child really close come the center, both children will be in perfect balance. If we could rotate the children roughly each other, both the huge child and the little child orbit a common center of mass.



In our solar system, every bodies orbit a common center of mass, consisting of the Sun, but the sunlight is so big in comparison to the planets, the center of mass in reality lies inside the Sun! This makes the sunlight seem to wiggle back and forth, and also the spectrum of the sunlight shifts ago and forth together well. We find for this spectral shift in various other stars to recognize if there space one or an ext planets orbiting the star. Once the star moves towards us, the irradiate emitted has actually a much shorter wavelength, so we say that is spectrum is blue shifted. As soon as it is relocating away indigenous us, the light has actually a much longer wavelength, so we say that is spectrum is red shifted. While the Doppler technique is many widely used for detecting extrasolar planets, the is best suited come look for really massive planets orbiting close come their parent star. This is due to the fact that the parent star will wiggle much more with a big planet nearby, thereby creating a larger and more easily detectable spectral shift. Most planets discovered approximately other stars have been an extremely massive and orbit extremely close to their parent star.

Another indirect observational tool is dubbed a transit. This is as soon as a planet crosses in prior of its parent star, temporarily decreasing the star"s brightness by a tiny amount. This method allows scientists to calculate the density and also size the the planet with respect come the size of the star.

Although these methods provide us one idea that size, distance, and also orbital period of the planets, they can"t really give us any concrete information about the planet. Straight observation is a far better tool than indirect, but due to the fact that these planets room so much away and essentially concealed from view because of how tiny and dim they are compared to their parental stars, straight observation is frequently impossible.

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Certain features are usual in most known exoplanets, and also the stars they room orbiting. Many of the stars that hold planets room main-sequence stars similar in spectral class to our own Sun. Most recognized exoplanets are fairly massive. This doesn"t average that Earth-sized exoplanets don"t exist—remember, ours detection approaches favor finding massive planets orbiting close to stars. Some also orbit much more closely than Mercury orbits the Sun. Unlike the practically circular orbits the the planets in our solar system, most exoplanets exhibit mainly eccentric orbits. Most of the recognized exoplanets space gaseous, comparable to the gigantic planets in our solar system, although some of the smaller exoplanets discovered show indications of rockier, terrestrial compositions.

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Scientists have discovered that most recognized exoplanets share countless similarities with the jovian planets in ours solar system, such together size, density, and also composition. Exoplanets are more than likely made that hydrogen and also helium gas. Due to the fact that these planets orbit near to your stars, your temperatures are more than likely much greater than temperatures on the jovian planets. If Jupiter orbited as closely as few of these exoplanets, theoretically it must actually be larger in radius due to the fact that Jupiter"s gas would warmth up and expand. Confirming this, we have actually observed a planet around star HD209458 the looks just like a puffed increase Jupiter. Scientists have referred to as these close-orbiting gas planets "hot Jupiters". Warm Jupiters probably have cloud layers, yet such high temperature would typical that different elements could condense. It would certainly be much too warm for ammonia, methane and also water to condense, choose they carry out in Jupiter"s atmosphere. This high temperatures could even enable for clouds make from products we would normally think of together solids on earth (for example, some metals).

Comparison between Jupiter and also a "hot Jupiter"(click come enlarge)

With the landslide that exoplanet discoveries in the last decade, researchers have started to question the theories us have around the development of our very own solar system. Exactly how is it the the planets in our solar system have such different features than the extrasolar planets we have found? The nebular theory states that the inner planets room rocky since rocks and metals condensation at high temperatures, when the external planets are greatly gaseous due to the fact that hydrogen compounds condensation at lower temperatures, for this reason farther native the Sun. Hot Jupiters, favor so countless of the known exoplanets, should not exist so close to your stars according to our model of formation, so just how did they form? Although it is not recognized for sure, one opportunity is the these hot Jupiters formed farther from their parent star and also migrated inward to a smaller, an ext eccentric orbit.

The knowledge of other planetary systems absolutely puts a glitch in our theory of formation, yet it doesn"t necessarily typical it"s wrong. Our theory works for us, but it is just that: a theory. It is very details to what us have found through centuries of research, and the centuries of study to come will surely change the means we perceive our world and also the universe.

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Image that the Milky means in the Cygnus region(click come enlarge)

The bulk of extrasolar planets recognize thus far are gas giants, like the outer planets in our solar system. Scientists are details that life, together we understand it, can not exist on the outer planets in our solar system. The an obstacle now is to discover terrestrial exoplanets that may be capable of maintain life. Reserved for launch in 2009, the Kepler Mission is designed to find our an ar of the Milky means for smaller, Earth-sized exoplanets in or close to the habitable zone of their parent star. Kepler will certainly stare in ~ a single portion of the sky in the Cygnus an ar along the Orion eight for its whole mission with hopes of detecting hundreds much more exoplanets.

There are particular conditions necessary to do a planet habitable, and there room a couple of key factors that influence these conditions. The size and temperature the the star, and also the orbit of the planet, largely determine the problem of having liquid water top top the surface. Planets also close come a star are so hot that any type of water on the surface ar would boil away, when planets too much from a star are so cold that any liquid water freezes. The size and mass of the planet determine whether or not it deserve to sustain one atmosphere. Smaller sized planets might not be substantial enough to have the surface ar gravity important to maintain an atmosphere. Various other factors, such as the composition of the atmosphere, will influence the temperature of the planet and also the kind of defense from various creates of harmful radiation.

In bespeak to find planets, Kepler will use the detection method known as a transit. A transit is an event in i m sorry a world passes in former of a star as checked out from Earth. Compared to a bigger planet, transits by terrestrial-sized planets develop a tiny change in the brightness the the star. This readjust in brightness lasts only a few hours, depending upon the distance in between the planet and the star. We understand a earth is causing the adjust in brightness if the adjust is periodic, which means that it happens again and also again top top the very same time period.

The main Kepler tool is a telescope dubbed a photometer, or light meter, through a very large field the view. It will stare at the exact same star ar for the entire mission and continuously and also simultaneously screen the brightness of more than 100,000 stars. If Earth-size planets space common, Kepler must detect hundreds of them.

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Want to know more about the Kepler mission? You deserve to learn more about the Kepler mission indigenous the official residence page.

RELATED LINKSLinks to lists of well-known extrasolar planets:Astrobiology WebCalifornia & Carnegie planet SearchJohnston"s Archive: Extrasolar Planets and also Brown Dwarfs