>The  planets is defined as a celestial body that is in orbit around the Sun which has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and has cleared the neighbourhood around its orbit.

Before 2006 there was no univocal definition of the planet , leaving to the general practice the task of identifying a planet in each celestial body with a certain significant mass and fixed on a definite orbit. On August 24, 2006, the [International Astronomical Union (UAI) provided the definition above](https://www.iau.org/news/pressreleases/detail/iau0603/).

The need for a clear definition was born, very recently, from the fact that based on the vague previous definition it was no longer possible to continue talking about nine planets in the Solar System: with the improvement of the observations we have come to identify hundreds of celestial bodies definable as planets because with characteristics similar to those of Pluto. It was necessary to convoke seven luminaries in astronomy to create a Committee aimed at providing a definition of the planet . According to this new definition, the Solar System has also passed from nine planets to eight, plundering its own Pluto among the dwarf planets . 

![](https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcRmpwX6DsARaMx-XriMG6pNfCeioylWfvs9NBrRr7dVA3AY4XCKB5NlShAN)
[Image Source](https://www.google.com.ng/amp/s/www.universetoday.com/34577/inner-and-outer-planets/amp/)

**From history of planets**
It is very recent, and has developed as a result of increasingly sophisticated technological inventions that allow you to explore distant space. From history, we know that the planets were identified with bodies that did not stand still in the sky, and that for this reason they were distinguished from fixed stars. We had, therefore, know that even the Moon and the Sun were considered as such, while Uranus, Neptune and Pluto were not even seen. It is not by chance that the term " planet " derives from the Greek planétes , which means "wandering" precisely in reference to the motion of these bodies with respect to the "fixed" stars. 

**Let's analyze the definition**
The planets they revolve around the sun (revolutionary motion) without being in turn. For precision, both the planet and the sun orbit around a common center of gravity, but the mass of the sun is so great that the center of gravity of the two orbits is almost at the center of the sun, so apparently the planet is rotating around the sun ( Universal Gravitation Law ). The spheroidal shape is due precisely to the rotation of the planets around their axis (called precisely axis of rotation). Usually the planets are spheroidal and crushed at the poles because of their rotation motion. In fact, by rotating, the speed of each part of a planet grows down from the North Pole to the equator (referring to the Earth) and it flows from the equator to the South Pole. Having to rotate as a single body, in fact, the widest part is forced to do it faster. This is not always true, but it also depends on the nature of the planet itself. For example, a rocky planet behaves as a solid body and rotates while gaseous planet can have different periods of rotation at various latitudes. 
It is important to bear in mind that the difference between a planet and a dwarf planet, even if the name suggests the opposite, is not at all in the dimension of the celestial body but in the presence or absence of all the characteristics that make up the definition of planet . To be precise, the lack of the ability to carve out a real orbit determines, in the presence of the other requirements, the classification among the dwarf planets.

The planetary classifications concern the composition or, in a more arbitrary from the scientific point of view, the belonging to the Solar System. Depending on the composition, the planets can be characterized by rock or gas: from this derives the classification in terrestrial (or rocky or telluric) planets and gaseous planets . On the basis of the belonging to the Solar System, instead, the planets are distinguished in solar and extra-solar (also called [exoplanets](https://en.m.wikipedia.org/wiki/Exoplanet)).


In October 2017, there are more than 3500 planets discovered around other stars and about 50 have dimensions between that of Mars and sometimes the size of the Earth and are placed in the habitable zone . Among these fifty, some may be "oceanic" planets, covered by water for a thickness of hundreds of kilometers ( water world), as seems to have been Earth and Venus during their evolutionary phases. So deep oceans provide a reservoir of water vapor for the atmosphere and so scientists try to calculate how stable an ocean and its atmosphere can be, especially as a function of solar wind and induced evaporation. Many of the 50 planets are in close orbit to their star, often of class M, so they are heavily exposed to radiation. The models carried out by the CfA show possible scenarios focusing on parameters such as magnetic fields, CMEs and atmospheric ionization. The results sometimes lead to planets like the Earth, so in line with the observations, but other times we reach a total loss of atmosphere: on average it seems that also aquatic worlds lose all their atmosphere around stars of class M, all in about one billion years ( [Astrophysical Journal, October 2017, CfA](http://iopscience.iop.org/article/10.3847/2041-8213/aa8a60/meta) )


### EXOPLANETS

>The  exoplanets  are non-stellar celestial bodies, orbiting around a star that is not the Sun, whose mass is sufficient to give it a spheroidal shape and whose orbital band is devoid of any bodies of comparable or superior dimensions.


The definition is the same valid for each planets , as evident, while what differentiates the exoplanets is that their star is not the Sun: in practice, the exoplanets are so named because they do not belong to our Solar System. Precisely for this feature are also called extra-solar planets .


The study of extra-solar planets is very recent, because only recently has it been possible to begin to see or to sense the presence of these bodies so far from us. One thing is to see a distant star because it shines from its part. Another thing is to see an exoplanet , much smaller and much less brilliant than its orbital fire. 
Recent history, as has already been said. The first exoplanet discovered, in fact, [is dated 1995](https://exoplanets.nasa.gov/the-search-for-life/exoplanets-101/) but it would be useless to say today how many have been discovered, since this would be updated continuously (an always updated database is represented by [exoplanet.eu](http://www.exoplanet.eu) website).

**Why are the extra-solar planets being sought?**
Perhaps the biggest motivation is a sense of loneliness. The Earth, objectively, has no special motives for which life must be present only in its territory, and in the infinite infinity of the universe there must also be a similar planet for conditions capable of developing life forms similar to ours. It is not the search for extra-terrestrial life that drives astronomers to this passion, but perhaps the branch of exoplanets is an exception, at least in part, to this rule. 
Despite the continuous increase in the number of exoplanets, it is not easy to find new ones and often we resort to methods that are not those represented by direct observation. It is no coincidence, in fact, that most of the exoplanets found features quite unusual compared to the planets to which we are accustomed in our planetary system : often associate larger dimensions than those of Jupiter, the largest solar planet, with very orbits tight around their star. We know that in the Solar System the narrowest orbit is that of Mercury , the smallest planet. Elsewhere, the opposite seems to be the case. Probably, however, it is only appearance: with our current means we can see only the planets directly larger, and if you do not see them directly you can see the effects that they produce. It is not that in other planetary systems there are no small planets : we simply cannot see them. Being able to see an exoplanet is complicated both for the very short distance (small angular distance) of the planet from the star and for the great difference in brightness of the two bodies themselves. Seen from so far, the two bodies are very close and the brightness of the star tends to cover the brightness reflected by the planet.


### EXOPLANET DISCOVERY TECHNIQUES
Till date, the planets identified outside the Solar System are comparable in size to those of our own gaseous giants. The current challenge is to look for terrestrial planets that can be potential cradles of extra-terrestrial life forms. A possible planet has already been found ( [Gliese 581c](https://amp.space.com/36196-gliese-581c.html) ). For the search for exoplanets exist various indirect detection methods, although right at the end of 2008 it started a new era: the direct vision of [four extrasolar](https://phys.org/news/2017-01-four-planet-imaged-motion.amp) planets around the beautiful star Fomalhaut and the least known star HR8799 in the constellation Pegasus .

Currently, the minimum mass of exoplanets found is that of Gliese 581c , equal to 5 or 6 land masses and placed around Gliese 581 , near Beta Librae . 
The majority of the exoplanets found, as mentioned, are of the hot Jupiters type , with high masses and low densities confirming the main composition of hydrogen and helium. An exception, however, is already known in the constellation of Eridanus , and it is the planet HD 149026b , a giant formed by rock and gas, more massive than Saturn but smaller. Half or two thirds of its mass should be present in the rocky and metallic form.
In 2008, a planet of about 22 land masses was discovered around a red dwarf star. This planet was called GJ 436b . Just GJ 436b and Gliese 581c could be planets very similar to Earth in terms of atmosphere. 
The COROT and Kepler probes will have the purpose of looking for exoplanets of terrestrial molds. 
The principle that our system is not special compared to others, has led us to think that " exoplanets"They exist in our galaxy but also in others, which is almost obvious: just in June 2009 it was accepted the publication of an [article](http://arxiv.org/PS_cache/arxiv/pdf/0906/0906.1050v1.pdf) that assigns to the gravitational pixel lensing method the discovery of a possible planet orbiting a galaxy star M31 ( Andromeda ): if the discovery was reiterated, it would be the first exoplanet discovered in a galaxy that is not the Milky Way, in fact the data are not current, but date back to 1999 with the difference that they are analyzed in light of new skills and of the new instruments available: the mass of the possible planetis at the limit between that of a large planet and that of a small star, so it is still all to be verified. 

Of all the exoplanets found, about 10% are in the habitability of the star but for life as we know there are no prospects, since they are gaseous giants. The discourse, recently, has therefore moved: do these exoplanets have moons? If so, perhaps life should be sought on these for now unobservable esolune .

![](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSaiVkQTMnJuj3DdWTqssfIWEXuF9gpnf2R8oipw6vaB13ablNAW53deg1JsA)
[Image  Source](http://www.manyworlds.space/index.php/tag/direct-imaging/)
**DIRECT VISION METHOD**
The method of direct vision, as mentioned, is very recent since until the end of 2008 no extra-solar planet had been seen orbiting around its star. 
The first was Fomalhaut b , around the very young and very bright star Fomalhaut in the constellation of the Austral Fish . There are four exoplanets photographed at the end of 2008, one around Fomalhaut and three around the star HR8799 in the constellation Pegasus .

![](http://wwwcdn.skyandtelescope.com/wp-content/uploads/Fomalhaut-b-system-NASA-and-ESA-ST.jpg)
[Image  Source](http://www.skyandtelescope.com/astronomy-blogs/exoplanets-wishful-thinkers10292014/)

Fomalhaut b has been photographed by a fantastic observational instrument that responds to the Hubble Space Telescope (HST) name: after eight years of careful observations we have come to photograph with certainty the planet b of the star Fomalhaut , also establishing the physical parameters and orbitals. What leaves people astonished is that the image was taken in the visible spectrum, while it was always believed that the most favorable band was infrared, to observe planets still warm in formation. Fomalhaut b resides in the dust band of the main star, which suggests that it may be a rocky planet or, at least, with a very important solid core.
The images were obtained by screening the light of the star with a special filter, so that its brightness does not obscure the much dimmer of the planet. 
 Another important factor: so far the stars of spectral class A (like Fomalhaut ) had been neglected because their extreme brightness made it almost impossible to see the brightness of any planets . Yet one aspect was left out: these stars have a very extended protoplanetary disk, which makes it possible to form planets in orbits very far from the central body.

![](https://www.universetoday.com/wp-content/uploads/2017/12/Radial-Velocity-Method-star-orbits-1024x878.png)
[Image Source](https://www.universetoday.com/138014/radial-velocity-method/amp/)

**RADIAL VELOCITY METHOD**
In the universe, the velocity of a star within the galaxy can be seen as constant. The presence of a planet near the star involves the presence of a gravitational field that alters the movement of the star itself. We know, in fact, that star and planet orbit around a common center of gravity, and only the imposing stellar mass causes this center of gravity to be shifted in favor of the star almost coinciding with its center (the consequence is that the star is almost still while the planet orbits them around). 
Spectrographic analysis of the star will therefore show changes in Doppler effect (or redshift)) that will allow the estimation of the mass of the planet and its orbital period. Observable effects, however, will only be present in the case of giant planets in close orbit. If someone observed the Sun from outside the Solar System and its only planet was Mercury, it certainly would not notice anything. With the radial velocity method in [1995 the first exoplanet was discovered , 51 Pegasi](https://www.universetoday.com/138014/radial-velocity-method/amp/). Subsequent exoplanets have been found for the most part with this method.

![](https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcShncznyqzszkIg4Z_N0FmBWK_2yYi-BKbHTs9me7Lq3Q8kdOP3DZ_TjkMOGg)
[Image Source](http://www.novacelestia.com/space_art_extrasolar_planets/detect_extrasolar_planets.html)
**ASTROMETRY**
Through accurate astronomical measurements it is possible to detect exoplanets by measuring from the star's own motion the orbital velocity around the common center of mass, perpendicular to the line of sight. The disadvantage is linked to the fact that the method can only be used for nearby stars and whose own motions are fixed in a very precise way: in practice the method consists in precisely measuring the position of a star and observing how much it changes in the time due to gravitational interaction with a companion object. Given that the change in position is very small, only recently (June 2009, with publication in The Astrophysical Journal of July 2009) was it possible to identify via astrometry an extrasolar planet and for a long time this method has remained aside. The planet has been discovered in orbit around an ultra-cold dwarf star. The planet has a mass of 6.4 MJ ( Jupiter Mass ), with an orbital period of 0.744 years around the star VB10 ( van Biesbroeck 1944 ), close to the lower limit mass for a star ( [see details](http://arxiv.org/ftp/arxiv/papers/0906/0906.0544.pdf)).


![](https://i.imgur.com/43f17Ke.gif)
[Source](https://imgur.com/r/SpaceGifs/43f17Ke)
### TRANSIT METHOD
The method of transits is the most recent: if a planet passes in front of a star it obscures the disk by a time fraction that is equal to the square of the ratio between the rays of the two celestial bodies. In practice, the star's light curve is observed, and where it is brighter the calculation is even easier. Also this time, of course, the best results are in cases of giant planets in close orbit. The great advantage is given by the fact that, by studying the obscuration of the star disk, one can also understand the radius of the planet and the inclination of the orbit with respect to the ecliptic. 
[The first planet to be discovered thanks to the transits method was, in 1999, HD209458B](https://en.m.wikipedia.org/wiki/HD_209458_b). One more information: the stellar spectrum that arrives directly when there are no intermediate planets shows certain absorptions and certain emissions within the spectrum. When the planet passes in front of the star, the spectrum can vary in terms of emissions and absorptions, which implies that the planet has some physical characteristics such as the presence of chemical elements such as sodium or hydrogen.

Transits require particular orbits, not long to be observed several times. [The planet HD76920b](https://theconversation.com/amp/weve-found-an-exo-planet-with-an-extraordinarily-eccentric-orbit-87011) has, for example, a very peculiar or very elongated orbit. It has four Jupiter masses and is in the Flying Fish at 587 light years from us. Its most distant point from its star is about 2 Astronomical Units.

![](https://ucrtoday.ucr.edu/wp-content/uploads/2017/11/HD76920bs-orbit.jpg)
[Image Source](https://ucrtoday.ucr.edu/50010/hd76920bs-orbit)

What displaces, even in terms of formation, is the orbit of this planet in terms of eccentricity that could be the consequence of the formation of two co-planar planets: one may have had the worst and been removed from the planetary system while the the other would be disposed in eccentric orbit. Alternative could come from the binary nature of the star. HD 76920b orbits an old star, about 7 billion years old, and experiences very strong tidal interactions at the periastrous point. It is a gaseous planet, so it should undergo strong deformations in the passage and this should lead to a circularization of the orbit over time, keeping the periastron fixed and bringing the aphis closer. Therefore, the planet should not be there since its birth. 

Sometimes the hot Jovian type planets do not pass completely in front of their star ( grazing transit , an example is [WASP-174b announced on February 2, 2018 on arXiv](https://arxiv.org/abs/1802.00766)). Its mass and its size are very similar to those of Jupiter but the temperature is much higher. The grazing transit are important because they allow us to better verify the presence of other celestial bodies by varying the transit times induced by gravitational perturbations. The discovery of WASP-74b was confirmed by Doppler effect observed by HARPS . Because of the type of transit the size of the planet is difficult to estimate but the value of WASP-174b should be between 0.7 and 1.7 Jovian rays.


![](https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcQT-di9V3fWmEJC79HPYzoq6oRVJRswp97vyX_1xQrGN5HI9Ci_vxsnJgvhCQ)
[Image Source](http://www.astronomy.com/News-Observing/News/2012/01/The%20Milky%20Way%20contains%20at%20least%20100%20billion%20planets%20according%20to%20survey.aspx)
**200+ planets for K2**
[Kepler's K2 mission](https://amp.space.com/39703-kepler-telescope-discovers-95-exoplanets-more.html) collects 100 additional exoplanets, from an analysis of 275 candidates. Of the 275, 149 were actually planets and 95 of these are new discoveries. The analysis still concerns the first release of K2 data, which was released in 2014. 
One of the new planets orbit a very bright star called HD 212657: it is an important discovery since it can teach a lot also through observations from Earth.


<div class="pull-left"> https://upload.wikimedia.org/wikipedia/commons/thumb/0/03/Imageedit_6_2578820362.gif/231px-Imageedit_6_2578820362.gif 
[Image Source](https://en.m.wikipedia.org/wiki/Gravitational_lens) </div>


**GRAVITATIONAL LENS**
[The gravitational lens of Einstein](https://en.m.wikipedia.org/wiki/Gravitational_lens) is not a method that was created to identify exoplanets , of course. However, under certain circumstances it can be interesting: a large planet would distort the ray of light emitted by a star (making it almost imperceptibly increase the brightness), and in this case it would be possible to identify more simply even the terrestrial planets . So far there have been no findings in this regard, however, and it is also easily explained. The effect, predicted by Einsteinin the Thirties, it concentrates on the observer for a short time the light rays of the star but the phenomenon is temporary since the star, planet and observer move in space and time, making the necessary alignment conditions disappear very soon. The event generates a typical light curve, known as the Paczynski curve , characterized by a rapid increase in brightness and a return to the initial value after a rapid maximum. 
In our Galaxy it is estimated that such an event could occur with a probability of one in a million for each star, given that a perfect alignment between star, planet and Earth is needed.
In reality, gravitational lenses, thanks to the tools that can be used today, have been discovered by the thousands but there is no proof of a star- planet combination , so to date no exoplanet has been discovered using this method.

With technology, the methods tend to increase, even in correspondence with predictive models: giant exoplanets in orbit far from their star are more likely to be discovered if around the star itself there is a disk of debris ( [Astronomical Journal, 2017 - IPAC / Caltech](http://www.spitzer.caltech.edu/news/1989-ssc2017-12-Giant-Exoplanet-Hunters-Look-for-Debris-Disks) ). The study focuses on the planets with more than five masses of Jupiter and is the largest, in terms of data, regarding the stars with dust disks: it would be the giant planets to keep these discs alive, so their presence indicates a star around which it is more likely to find giant planets. The probability is nine times higher and the result is the result of the data obtained by Spitzer on 130 single stars with disk and 277 stars without disk, with ages between a few million years and one billion years. The link between giants and disks is only statistical, the reason has not yet been found by any theory even if intuitively the gravity exerted could increase the number of collisions between planetesimals. If this were the case, similar planetary systems could also present a total absence of smaller planets. There are examples that confirm but also examples that do not: the planet of Beta Pictoris, for example, is in a system with a disk.


A massive exoplanet resulted from data from the gravitational microlensing survey of Spitzer Space Telescope . The planet is in the galactic bulge and is called OGLE-2016-BLG-1190Lb and is the first discovered by Spitzer in this way and in this area ( [October 27, arXiv](https://arxiv.org/abs/1710.09974)).
The star was discovered in June 2016 through a microlens event from the Optical Gravitational Lensing Experiment ( OGLE ), a Polish project of the University of Warsaw that looks for exoplanets and dark matter through a 1.3-meter telescope at Las Campanas in Chile. 
A few days later, Spitzer observed the same area by uncovering a new object around the dwarf star. The mass is about 13.4 Jupiter masses, at the limit of the fusion of deuterium and therefore at the limit of the planet-star boundary. It could still be a brown dwarf, given the uncertainty. The distance from the star is 2 UA. The mother star is class G and has a mass of 0.89 solar masses, 22 thousand light years away from us.

![](https://encrypted-tbn3.gstatic.com/images?q=tbn:ANd9GcSZ_jUpdxvX9W6V2i34rigQ65v4HzWd4LtmnAY0dZnrSKPO85xRWgYRj80wPQ)
[Image Source](https://www.google.com.ng/amp/s/phys.org/news/2017-11-extremely-massive-exoplanet-milky-bulge.amp)

In 2016, the first measurement of parallax microlensing of a small stellar object was carried out using Spitzer's observations and a terrestrial telescope, but a complication was not infrequently linked to the fact of using only two observation points. Three points would have eliminated the uncertainties and so a CfA team led by Jennifer Yee , in 2017, observed the first gravitational microlens event from three different vantage points : Spitzer, the Earth and the Kepler telescope (K2 mission). The object is MOA-2016-BLG-290 and is a star of just 0.07 solar masses placed at 22 thousand light years away from us ( [Astrophysical Journal, December 2017, CfA](http://iopscience.iop.org/article/10.3847/2041-8213/aa93fa/meta)).


![](https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcQmNLTKhOySLmdysNg_coWpWMQjrMNpOcm2Z9w9YpJnZ-QUS2QI0GYHfKTWRw)
[Image Source](https://www.google.com.ng/amp/s/phys.org/news/2017-12-microlensing-event-positions-space.amp)

Exoplanets, very "eso", were hypothesized at the [beginning of 2018](http://iopscience.iop.org/article/10.3847/2041-8213/aaa5fb/meta) through a phenomenon of gravitational microlens : these would be planets belonging to another galaxy, with masses ranging from the mass of the Moon to that of Jupiter. The data are those obtained by Chandra X-ray Observatory , a space telescope controlled by the Smithsonian Astrophysical Observatory , with the analysis carried out by the Supercomputing Center for Education and Research . This is an example of how powerful this technique of analysis is: the galaxy is 3.8 billion light years and obviously there is no possibility to directly observe these planets.


**REASON TO THE CONTRARY**

![](https://3c1703fe8d.site.internapcdn.net/newman/csz/news/800/2017/arewebeingwa.png)
[Image Source](https://m.phys.org/news/2017-09-tens-worlds-earth.html)

A team from Queen's University in Belfast and the Max Planck Institute for Solar System Research in Germany have thought otherwise: which of the exoplanets discovered to date could, in turn, discover the Earth during a transit on the Sun? The article, published in [Monthly Notices of the Royal Astronomical Society in September 2017](https://academic.oup.com/mnras/article-abstract/473/1/345/4082231?redirectedFrom=fulltext), indicates at least nine exoplanets able to discover us. One of the results, surprising if we want, sees the terrestrial planets easier to discover than the gaseous giants of our Solar System, despite its small size: not the size that counts but the proximity of a planet to its star, and this goes to favor of the Earth and its rocky brothers. 
A random observer in the Galaxy would have a probability of 40 to discover at least one planet around the Sun. The probability of discovering two planets is ten times smaller, while that of discovering three planets would be ten times lower . 
Among the thousands of exoplanets discovered to date, about 68 could see one or more solar planets in transit. Nine of the sixty-eight are in an ideal position to see the Earth pass even if none of these nine is considered habitable. 
Statistically there should be, however, ten habitable planets (still undiscovered) able to observe the Earth in transit.


### EXOATMOSPHERE

One of the challenges of the beginning of the new millennium, with the discovery of numerous exoplanets, consists in characterizing the atmospheres and for this most important purpose of the mass is the dimension of a planet, at least according to a [September 2017](http://meetingorganizer.copernicus.org/EPSC2017/EPSC2017-389.pdf) study [presented at the European Planetary Science Congress](http://meetingorganizer.copernicus.org/EPSC2017/EPSC2017-389.pdf). The analysis of 30 exoplanets led to this conclusion, succeeding in characterizing the atmosphere of 16 "hot Jupiters" and revealing water vapor in each case. The study proves essential to be able to obtain information on each of the planets discovered today and to achieve the result has processed the data of WFC3 aboard the Hubble Space Telescope, obtaining the spectral data of 30 exoplanets. As mentioned, to observe the exo-atmosphere does not count the mass of an exoplanet but its size, with the implication that the gravitational force of the exoplanet is actually a minor effect in this sense. 

Many of the studied atmospheres show the presence of clouds while the warmer planets, with temperatures exceeding 1700 ° C, seem to have clear skies at least at high altitudes. In these planets, in addition to water vapor, titanium oxide and vanadium oxide have been traced.

![](https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRzAeFQ5S_yuE0mMF3M0U74j-FbT6WeNb9-S52uhBI_sl4kkVQHfXTgcUFW)
[Image Source](http://www.europlanet-eu.org/size-matters-in-the-detection-of-exoplanet-atmospheres/)

It snows titanium oxide on Kepler-13Ab , according to the Hubble Space Telescope . The process is known as the " cold trap " and is the first time it is observed on the stage of the universe ( [The Astronomical Journal - Penn State University](http://iopscience.iop.org/article/10.3847/1538-3881/aa899b/meta)). The study of exoatmosphere is an important step for the determination of potentially habitable planets and it does not matter if you start from the study of non-habitable gaseous giants because experience and technologies will increase up to be applied on rocky exoplanets similar to the Earth in the future. 
Kepler-13Ab  is one of the hottest known exoplanets, with a diurnal temperature of 5000 Fahrenheit and a rotation blocked by stellar gravity. Snow is present only on the night side. The planet's atmosphere is much colder at high altitudes, which is surprising compared to other hot jupiter trends. Titan oxide in the atmosphere of other similar planets absorbs stellar light and radiates it like heat, making the atmosphere warmer as it rises to altitude. Intrigued by this anomaly, scientists have concluded that the star-shaped form of light-absorbing titanium is removed from the day side of the planet, and this absence affects the falling temperature.
The powerful sells on the planet carry titanium oxide, condensing it into crystal flakes that form clouds. The gravity exerted by the planet, six times greater than that of Jupiter, then determines the fall of titanium oxide in the form of snow from the upper atmosphere to a lower layer, on the night side. The theory is valid for years but it would be the first time that the phenomenon becomes evident. The titanium does not move so far from the point of fall, so it returns in gaseous form in the day part. Hubble observed everything in the near infrared during the passage of the planet behind it star, in a secondary eclipse.



![](https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRzh_FAVoJNoFKHVFBozIcs4nVHazUQ5ri-rJQ1LGC0t_B47bN5j6myIyr3UA)
[Image Source](https://www.google.com.ng/amp/www.dailymail.co.uk/sciencetech/article-5084687/amp/Astronomers-planet-host-alien-life.html)

A planet 11 light years away from us could have a fairly mild climate, similar to the Earth's. The planet is called [Ross 128b](https://en.m.wikipedia.org/wiki/Ross_128_b) and was discovered using the Harps tool ( High Accuracy Radial velocity Planet Searcher)) around a non-active red dwarf, and it is precisely the lack of activity that increases the chances of sustaining life forms. The planet's orbital period is 9.9 days and is of small mass, with a surface temperature similar to that of Earth. The absence of powerful and typical flares of red dwarfs would make the "life" of the planets much more serene. The star's own motion will ensure that this planet, among those discovered, will be the closest to ours in 79 thousand years, thus approaching more than today Proxima b. The planet orbits 20 times closer to the star than does the Earth with the Sun but receives a radiation equal to 1.38 times that of the Earth, with an equilibrium temperature estimated between -60 and + 20 ° C ( [Astronomy and Astrophysics](https://www.eso.org/public/archives/releases/sciencepapers/eso1736/eso1736a.pdf)) . 
WASP-18b , on the other hand, seems to be surrounded by a [stratosphere charged with carbon monoxide but without water](https://www.nasa.gov/feature/goddard/2017/wasp-18b-has-smothering-stratosphere-without-water). A stratosphere is formed from particles that absorb UV and visible radiation by releasing energy as heat. WASP-18b orbits very close to the star-mother and has an unusual composition that suggests a formation different from that of gaseous giants like Jupiter. It is 325 light years from Earth and its peculiarity is the dominance of carbon monoxide, which opens the way to new physical processes in planetary atmospheres. To be precise, WASP-18bit has hot carbon monoxide in the stratosphere and colder in the underlying layers, then in the troposphere. To get to the spectroscopic signatures obtained, the high atmosphere of WASP-18b must be charged with carbon monoxide with a very high metallicity: its formation must therefore have been very different from that of Jupiter.


![](https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcRiXgwT-JVXTYlcJy1QuMDZRgYhS9xrOEYNnS8cGjPqgph-IkQTutO9SOtB)
[Image Source](https://www.google.com.ng/amp/s/scitechdaily.com/astronomers-unlock-the-mysteries-of-super-earths/amp/)
**How do you understand if a planet is rocky or gaseous?**
One of the most massive and dense super-Earth could be around the star [GJ 9827](https://arxiv.org/abs/1711.01359) , 100 light-years away in the direction of Pisces. The planets could be three, as indicated by the Kepler / K2 space telescope, with all the largest components on Earth. Our Solar System is totally free of such planets. 
Radius and mass can provide an indication on the density and therefore on the rocky or gaseous nature of the planets and the measurements provide a sort of trend: planets with radii greater than 1.7 terrestrial rays have a gaseous shell similar to that of Neptune while the planets with minor rays are rocky like the Earth itself. 
The difference should be in the photo-evaporation process, which tears the volatile substances to the planets, creating bodies of smaller radius (hypothesis, however, under test). The three planets of GJ 9827 have rays of 1.64, 1.29 and 2.08 terrestrial rays, thus embracing the range indicated for this type separation. The system was then monitored by Planet Finding Spectrograph ( PFS) to have a range of possible masses. Planet b, the first, which count eight terrestrial masses, making it one of the densest ever discovered in terms of SuperTerre, with a composition given to about 50% iron. The masses of the planets ced are about 2.5 and 4 times the earth's mass respectively, even if the uncertainty is high. The planet d could therefore have a volatile shell very often while for the planet c it is not yet possible to draw conclusions.


![](https://lh6.googleusercontent.com/proxy/RAxI2ZkA5Q4Qj34d09g8wXEiYuH_JT4N3Y7Jin48KTiqo0r9bVQBRh1bT4Gd-yP_PmBA1uBVC4Lr6w0vl2DimOT7C9uzbJhCnXVaAKLDQpHWG_LVwgnEZV-Ijbe02f7qBoiIKmLIp-_YXAf04nbuw__cXA6P5dFVWjjVsaZKjszJixnnSR-ZsRM=w512-h288-nc)
[Image Source](https://sciencesprings.wordpress.com/tag/hot-jupiters/)
**Hot Jupiter**
The hottest point of a hot Jupiter gas planet is not where one would expect to find it, which poses a new challenge to paleontologists. The hottest point of a similar planet that always faces the same face to the star is undoubtedly the point closest to the star itself, but in reality can be developed equatorial winds that can "move" this hot spot to the east. In the case of [CoRot-2b](http://exoplanet.eu/catalog/corot-2_b/) , instead, the hot spot is in the opposite direction, to the west with respect to the center and therefore against any theory and expectation. On [CoRot-2b](https://www.nature.com/articles/s41550-017-0351-6) therefore, the winds blow in the opposite direction. There must be something unusual in the atmosphere of this planet and the use of Spitzer allowed to observe an entire orbit around the star by analyzing the surface of the planet for the first time, discovering and confirming the reverse motion of the winds.

**There are three possible explanations, but each of them opens further questions:**

* The planet could rotate so slowly that a rotation takes longer than a revolution, which would create winds to the west. The problem, however, lies in the violation of theories about gravitational interaction in such tight orbits;


* The atmosphere could interact with the planetary magnetic field and modify the patterns, which would provide a rare opportunity to study the magnetic field of an exoplanet; 


* The presence of large clouds covers the area east of the planet to make it darker than it actually would be, but this would be against atmospheric models on this type of planet.


![](https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcRfkQec_PnWmp1qFf7e6OXBCl5icpnhFyeNmbryfqrt5VVwpzNBl7Ym45Yw2g)
[Image Source](https://www.nytimes.com/2017/12/14/science/eight-planets-star-system.html)
**Kepler-90**
A solar system with eight planets was announced by the [University of Texas, Austin, on December 14, 2017](https://www.nasa.gov/press-release/artificial-intelligence-nasa-data-used-to-discover-eighth-planet-circling-distant-star). The star that hosts the planets is Kepler-90 . The eighth planet , Kepler-90i, is a hot and rocky planet that orbits its star in 14.4 days and was discovered thanks to computers able to "learn" to discover planets from Kepler's data and its planetary transits .  


https://youtu.be/S_HRh0ZynjE
A Short Description From NASA

A neural network, therefore, an intelligent system that self-learns as Kepler's data flows and has identified the faint signals of a still undiscovered planet around the star Kepler-90, a solar-type star 2545 light-years distant in the constellation of the Dragon.

From the point of view of the search for life, this system is not of great importance, since the eight planets are all closer to their star than the Earth is from the Sun. Kepler-90i is 30% larger than the Earth and it is so close to its star that the average temperature exceeds 800 degrees Fahrenheit, like Mercury. The outermost planet is a gas giant, the planet h, big like Jupiter and with a year of 331.6 Earth days. It is a mini-Solar System: large and small planets but all in very small scale in terms of distances.

Kepler-90 was the first planetary system to count seven planets as early as 2013 but the eighth had a signal too low to be tracked down by "classic" technologies. The same neural network has found a sixth planet in Kepler-80, called h, which forms a resonance chain with four others. ([see details](https://www.cfa.harvard.edu/~avanderb/kepler90i.pdf)).


![](https://www.sciencedaily.com/images/2017/12/171218120234_1_540x360.jpg)
[Image Source](https://www.sciencedaily.com/releases/2017/12/171218120234.htm)

### Proxima Centauri

[Proxima Centauri](https://en.m.wikipedia.org/wiki/Proxima_Centauri) continues to be the most studied star system, given its distance, and the presence of planets has led to an ever-increasing number of articles since the discovery of the first exoplanet of the system itself. In the face of models that speak of the possible presence of several giant planets, one last glance at the end of 2017 seems to disprove this possibility to support the thesis that sees the small planets as the most common type in the Universe. 

[New data have come from the most advanced spectrographic tools](https://www.sciencedaily.com/releases/2017/12/171218120234.htm) located in Chile: CHIRON , HARPS and UVES . A system of telescopes to try to answer the question " If there was a rocky planet in Alpha Centauri A and B, could we find out? " The answer has always been "No" but some says that around Alpha Centauri A there may be more planets small ones of 50 terrestrial masses while for Alfa Centauri B there could be planets smaller than 8 terrestrial masses. For Proxima , however, there may be planets less than one and a half times the land mass. Planets in Jupiter format would not be present.

TO BE CONTINUED !!!!

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Big Thanks to @enginewitty for this cool Banner


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