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An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun. Thousands of possible exoplanets have been found through ground-based and space-based observatories. Over 2000 exoplanets have been discovered since 1988 (more specifically, 2108 planets in 1350 planetary systems, including 511 multiple planetary systems, have been confirmed, as of 20 April 2016).

These exo-worlds come in a huge variety of sizes and orbits. Some are gigantic planets hugging close to their parent stars; others are icy, some rocky. One or two even were found to have their own moons! Space agencies from around the world are looking for a special kind of planet: one that’s the same size as Earth, orbiting a sun-like star in the habitable zone. Why? A habitable zone planet has the potential to support life!

The habitable zone is the range of distances from a star where a planet’s temperature* allows liquid water oceans, critical for life on Earth. The earliest definition of the zone was based on simple thermal equilibrium, but current calculations of the habitable zone include many other factors, including the greenhouse effect of a planet’s atmosphere. Its magnetic field (1). Its plate tectonics (2). Its life itself (3)! And several other conditions (4).This makes the boundaries of a habitable zone "fuzzy." Therefore, scientists are now realizing that 'The Goldilocks Zone' that  has long been defined as the band of space around a star that is not too warm, not too cold, rocky and with the right conditions for maintaining surface water and a breathable atmosphere , which to date scientists have only been able to calibrate using observations from our own solar system, may be too limiting. And they think the planet-formation can take different routes and can be dissimilar to the one of our solar system. So 'Habitable Planets' may lie outside the "Goldilocks zone" in extra-solar systems, and that planets farther from or closer to their suns than Earth may harbor the conditions necessary for life.

Protoplanetary disks are disks of gas and dust where planets form over the course of millions of years.

Now as you can see the exo-planets will be several light years away from us. And they will be very tiny when compared to stars. Moreover, the star light can completely subdue them. So how can anybody detect these 'small objects' from Earth?

Well, if the scientists have the will, can they keep themselves away from finding ways? The scientific creativity works here at its best. Here is how the scientists go about it...

There are two basic search strategies:

Direct Detection

Take pictures of planets orbiting other stars
Observe the transits of planets across the disks of their parent stars, which causes a characteristic drop in brightness.

Recently a shadow spotted by NASAs Hubble telescope sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding a young star may point to a new planet located 192 light-years away. Although the planet itself is not casting the shadow, it is doing some heavy lifting by gravitationally pulling on material near the star and warping the inner part of the disk.

The twisted, misaligned inner disk is casting its shadow across the surface of the outer disk. Although not certain, scientists think this shadow play points most probably to a new planet.

Gravitational Detection

Orbital motions ("wobbling") of the star because of the planet's gravity.
Gravitational microlensing of a background star by the planet.


Most exoplanets can only be detected indirectly because bright light from the stars that they orbit drowns them out. Dimming of star light when a planet passes before it can be used. A technique called the “transit” method, measuring how much a star's light dims when a planet passes in front of it is the way followed by astronomers. If the orbital plane of an extrasolar planet is aligned with the line of sight then the planet will periodically cross ("transit") the face of its parent star. This dims the star by 1% or so during the transit. This method requires precision photometry.

Another method is to look for tiny wobbles in stars' positions caused by their gravitational interactions with orbiting planets. Before the era of exoplanet discoveries, instruments could only measure stellar motions down to a kilometer per second, too imprecise to detect a wobble due to a planet.

Planets orbit on ellipses with the center of mass at one focus. The star also orbits around the planet-star center of mass, but much closer to the center of mass at a slower orbital speed because of its greater mass. Viewed from afar, the star will appear to wobble about the center of mass of the star-planet system. There are two manifestations of this motion: Astrometric Wobble (Parent star wobbles back & forth on the sky as seen relative to distant background stars) and Doppler Wobble (star's spectral absorption lines shift towards the blue when the wobble moves the star towards the Earth and Star's spectrum shifts towards the red when the wobble moves the star away from the Earth).

Microlensing is based on the gravitational lense effect. A massive object (the lens) will bend the light of a bright background object (the source). This can generate multiple distorted, magnified, and brightened images of the background source. When a distant star gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object. Since microlensing observations do not rely on radiation received from the lens object, this effect therefore allows astronomers to study massive objects no matter how faint.

Gravitational microlensing: This is how it works...

Picture credit : Google 

A “verification by multiplicity” method is the one that should increase the rate at which astronomers promote candidate planets to confirmed planets. The technique is based on orbital stability — many transits of a star occurring with short periods can only be due to planets in small orbits, since multiply eclipsing stars that might mimic would gravitationally eject each other from the system in just a few million years.

Space missions will soon pick up the transit search from space. From the ground, the HARPS spectrograph on the European Southern Observatory's La Silla 3.6-meter telescope in Chile is leading the Doppler wobble search in finding exo-planets.

It is sensitive in the infrared, it can sense the temperature profile of an exoplanet and give insights into its atmosphere -

Understanding protoplanetary disks can help us understand some of the mysteries about exoplanets, the planets in solar systems outside our own. Researchers use a method called "photo-reverberation," also known as "light echoes." When the central star brightens, some of the light hits the surrounding disk, causing a delayed “echo.” Scientists measure the time it takes for light coming directly from the star to reach Earth, then wait for its echo to arrive.

According to Albert Einstein's theory of special relativity, light travels at a constant speed. To determine a given distance, astronomers can multiply the speed of light by the time light takes to get from one point to another. To take advantage of this formula, scientists need to find a star with variable emission -- that is, a star that emits radiation in an unpredictable, uneven manner. Our own sun has a fairly stable emission, but a variable star would have unique, detectable changes in radiation that could be used for picking up corresponding light echoes. Young stars, which have variable emission, are the best candidates to study in this way. This new approach can be used for young stars with planets in the process of forming in a disk around them.

Sometimes light just gets in the way to detect exo-planets. A look at two technologies that block starlight to give telescopes a better view of distant Earth-like planets:

All this is so exciting. Very soon we might find our cousins in other solar systems! Get ready to interact with them. And they need not be like anything we ever imagined! Who knows how many surprises are waiting to be discovered?

* Astronomers have re-examined the possibilities for "habitable zones," or "Goldilocks zones," surrounding alien stars. Researchers found that habitable planets can exist in orbits closer to their parent stars than previously believed, because the solar energy required to start a runaway greenhouse effect is higher than was thought.

Too close to a star, an otherwise pleasant planet develops a runaway greenhouse effect, a feedback loop that leads to extremely high surface temperatures. The oceans boil, becoming thick clouds of vapor in the atmosphere. The thick atmosphere traps solar heat on the planet’s surface. 

The greenhouse effect traps infrared rays (heat) from the sun in the atmosphere, raising surface temperatures. Despite having a similar size to Earth, the planet venus is entirely uninhabitable due to its runaway greenhouse effect.
 
Over time, the sun’s brightness increases, pushing back the inner boundary of the habitable zone. Currently, the inner boundary is 95 percent of the distance from the sun to the Earth. In a billion years, the Earth may develop a runaway greenhouse effect like Venus'.

A new study suggests that, contrary to prevailing wisdom, the temperature of a planet doesn't always stabilize over time, so hot-blooded worlds may have a hard time holding onto liquid water — even if they reside in the temperate region around their stars known as the habitable zone (5).

So being in the habitable zone is not sufficient to expect Earth-like planetary evolution. Even if you place a planet with the Earth-like chemical composition — the right amount of water, and so on — it may not evolve Earth-like if it started out too hot or too cold (6).

References:

1. http://kkartlab.in/group/some-science/forum/topics/the-importance-o...

2. https://www.sciencenews.org/article/exoplanets-need-right-stuff-be-...

3. http://www.scientificamerican.com/article/which-came-first-on-earth...

4. http://www.pnas.org/content/early/2016/06/29/1523233113

5. http://www.space.com/23910-habitable-zones-alien-planets-stars-info...

6. http://advances.sciencemag.org/content/2/8/e1601168

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http://www.scientificamerican.com/article/which-came-first-on-earth...

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Our solar system is exceptional because of one thing, Saturn. In the vast majority of solar systems the inner rocky planets are pushed into the parent star by the Jupiter equivalents. The “Hot Jupiters” as they are called then eventually merge with the parent star as well leaving the ice giants similar to Neptune and Uranus as what we first started calling “Super Earths” behind. Saturn stopped this process from happening by creating a tug on Jupiter preventing it from its suicidal march into the sun.

We are exceptional because we have long lived rocky planets. That is a very rare thing in the Universe.

Hellish Venus Might Have Been Habitable for Billions of Years

A team of astronomers think the torrid and toxic world was once a cozy home for potential life

http://www.scientificamerican.com/article/hellish-venus-might-have-...

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Hydrogen volcanoes may increase habitability of exoplanets

Does TRAPPIST-1 have three potentially habitable planets or four? New research by scientists at Cornell University led by Ramses Ramirez, indicates that number four is a possibility if there are hydrogen volcanoes on it. According to the astronomers, worlds where hydrogen spews out from volcanic vents could enjoy a greenhouse effect that would warm their atmosphere enough to sustain life.

When the discovery of seven Earthlike planets with three occupying the habitable zone of the Jupiter-sized red dwarf star TRAPPIST-1 was announced last week, it caused a minor sensation in the scientific world. The habitable, or Goldilocks, zone is that band of orbits where it is not too hot and not too cold, but just right for a planet to potentially have liquid water on its surface.

In our solar system, the habitable zone extends from about the orbit of Venus to slightly beyond that of Mars, so three planets in the system could potentially support life and one definitely does. While exoplanets orbiting other stars have been found in their local habitable zone, finding seven Earth-like planets revolving around a small, cold dwarf star with three in the right place is the astronomical equivalent of drawing to an inside straight.

Not surprisingly, this makes scientists think that there may be more habitable planets than previously believed and the new Cornell study indicates that the habitable zone of TRAPPIST-1 and other stars may extend farther out than previously thought.

One thing that keeps the Earth at a habitable temperature is the greenhouse effect of water vapor, carbon dioxide, and other gases in the atmosphere. Remove their effect and the surface temperatures would be more like Mars. According to Cornell, if an otherwise icy exoplanet had an atmosphere of hydrogen, carbon dioxide, and water vapor, it, too, would enjoy a greenhouse effect, which could warm it enough for liquid water to exist and effectively extend the habitable zone by 30 to 60 percent. In addition, the planet would contain biosignatures, such as methane or ozone, in its atmosphere that our next generation of space telescopes could detect.

The only snag is that hydrogen is a gas that does not like to stick around. In our solar system, only gas giants like Jupiter have enough gravity to keep most of their hydrogen in the atmosphere. On small rocky worlds like Earth, any hydrogen simply floats up and off into space. Cornell's research indicates that if an Earth-like planet had volcanoes that belched out hydrogen at a steady rate, it could maintain a high enough level of the gas in the atmosphere to sustain a greenhouse effect.

If such a planet existed in our solar system, it could orbit as far as 2.4 times the distance from the Sun as the Earth, which would place it roughly in the asteroid belt between Mars and Jupiter. For TRAPPIST-1, if the outermost seventh planet has hydrogen volcanoes, it means that the system has four potentially habitable worlds instead of three.

"Finding multiple planets in the habitable zone of their host star is a great discovery because it means that there can be even more potentially habitable planets per star than we thought," says Lisa Kaltenegger, Cornell professor of astronomy and director of the Carl Sagan Institute. "Finding more rocky planets in the habitable zone – per star – increases our odds of finding life."

The results were published in The Astrophysical Journal Letters,

Source:
Cornell University

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