Exoplanets

Article by Mansha Kochar

Space exploration has always been an intriguing topic of interest for human beings. From understanding the creation of the Universe to the formation and calculation of the age of various celestial bodies, their respective behaviours and compositions (in the form of emission spectrums), scientists and astronomers have always been on their curiosity voyage, looking out for more information with respect to various elements of the Universe, in order to come up with explanations for their respective behaviours.

One of the prominent motivating factors for further exploration in the sphere of planetary science is the possibility of the existence of life on other planets. There has been considerable research on the topic considering the planets in our own solar system; however, the research has been fueled by the discovery of the existence of planets belonging to other solar systems, known as exoplanets.

WHAT IS AN EXOPLANET?

An exoplanet or extrasolar planet is a planet outside the Solar System (refers to the particular solar system in which Earth is a planet). According to the International Astronomical Unit (IAU), an exoplanet is defined as “Objects with true masses below the limiting mass for thermonuclear fusion of deuterium (currently calculated to be 13 Jupiter masses for objects of solar metallicity) that orbit stars, brown dwarfs or stellar remnants and that have a mass ratio with the central object below the L4/L5 instability \((\frac{M}{M_{central}} < \frac{2}{(25+\sqrt{621}})\).” Substellar objects with true masses above the limiting mass for thermonuclear fusion of deuterium are “brown dwarfs”, no matter how they formed or where they are located.

An alternate distinction between the two is suggested on the basis of formation. It is widely thought that giant planets form through core accretion, which may sometimes produce planets with masses above the deuterium fusion threshold. Brown dwarfs form like stars from the direct gravitational collapse of clouds of gas and this formation mechanism also produces objects that are below the 13 $M_{Jup}$ limit and can be as low as 1 $M_{Jup}$. While some other organisations such as the Extrasolar Planets Encyclopaedia have included objects upto 60 Jupiter masses.

HISTORY OF EXOPLANET DISCOVERY

The first evidence of a possible exoplanet, orbiting Van Maanen 2, was noted in 1917, but the first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmation of detection came in 1992 from the Arecibo Observatory, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. On 21st March 2022, the 5000th exoplanet beyond the Solar System was confirmed.On 11 January 2023, NASA scientists reported the detection of LHS 475 b, the first exoplanet discovered by the James Webb Space Telescope.

HOW DO WE DETECT THE EXISTENCE OF AN EXOPLANET?

  • Radial Velocity:

    It is one of the first successful and most efficient ways of discovering the possible existence of an exoplanet. It is based on the principle of ‘Doppler shift’. The proposed argument is that the way a planet is caused to revolve around a star due to the gravitational pull of the star, in a similar manner, the star also moves around because of the gravitational pull by the planet.

Since the planet is considerably smaller than the star in size, hence its movement is more apparent while on the other hand, the star movement is quite less and hence almost unperceivable by simple, crude observation. However, the movement can be detected by the changes in the wavelength of the energy waves emitted by the star, which is known as the ‘Doppler shift’. Hence, by studying the changes in these wavelengths, we can get an idea of the possible existence of exoplanets, and also an estimate of their size and number, if present.

  • Transit Method:

    This method is similar to the concept of eclipses; when a planet passes directly between an observer and the star it orbits, it blocks some of that star’s light. For a brief period of time, that star actually gets dimmer. This helps in identifying the possibility of planets revolving around that star. In addition to this, the size and length of the transit gives an idea of the size and distance of the planet; large planets will block light, and likewise, planets farther away from the star will take longer time to pass in front of it.

In a similar way of understanding, we can also estimate the number of planets revolving around a star. In case of more than one planet transiting a star, the light curves will get complicated instead of simple ones due to overlapping effects of the planet.

In addition to aiding the discovery of planets, this method can also be used to obtain information about their atmospheric composition and temperature. This is achieved by analysing the spectrum disturbance in the transit period.

  • Direct Imaging:

Although it is exceedingly difficult to directly capture images of exoplanets owing to their faraway distances and the fact their brightness is a lot times less than the star of their own solar system, new techniques employing the use of advanced technological developments have made it comparatively easier. However, the primary problem is posed by the star of the solar system, which overshadows the brightness and/or the emitted radiation by the planets orbiting it. In order to counter this, there have been various techniques to build instruments to block out the light of the host stars in the interest of getting a better look at the planets around them. There are two methods prominently used by astronomers for this purpose: Cornographs: These are internal add-ons to telescopes, which block the light from reaching the telescope detector. They are used in ground-based telescopes to directly image exoplanets. Starshade: It is a device positioned in such a way as to block light from a star preventing it from entering a telescope. They are predominantly in the form of separate spacecraft, positioned just at the right distance and angle with respect to the space-based telescope used by the astronomers.

  • Gravitational Microlensing:

The phenomenon of the focusing of the light from a distant star, due to any object, such as a planet or another star, causing it to appear temporarily brighter is known as gravitational microlensing (depicted by the animation below, along with the change of brightness).

While observing the event, it appears like a star getting gradually brighter over a certain period of time (like a month or so), before it gets dimmer. For a planet, it appears as a brief blip of light happening over the process. There is no concrete method for astronomers to predict these lensing events. Hence, they are required to watch over large parts of the sky, over a long period of time. On obtaining any records of such events, the data is analysed for obtaining detailed information about the star.

  • Astrometry:

As discussed above, the planets in a solar system also cause their local star to wobble due to the gravitational force, which causes a wavelength shift in the emission spectrum of the star. This method is more preferred than measuring actual wobble since it is far more challenging to witness such a minute difference in the position of the star. However, it is not entirely impossible to achieve.

This method, of tracking the actual movement of these stars, is called astrometry and consists of taking a series of images of a star and some of the other stars that are near it in the sky. In each picture, the distances between these reference stars and the star being checked for exoplanets is compared. If there is any movement recorded with respect to the other stars, it can be analysed for signs of movements. However, it is to be kept in mind that this method requires extremely precise optics, and the Earth’s atmosphere also poses a challenge in case of ground observations.

TYPES OF EXOPLANETS

With a wide range of exoplanets varying in size, surface type, surface temperature and many such properties, scientists have categorised these exoplanets in four major categories based on composition, each planet type varying in interior and exterior appearance:

  • Gas giants:

These are planets large in size, mostly composed of helium and/or hydrogen. They don’t have a hard surface, and instead have gases swirling around a solid, rocky core. Examples include Jupiter and Saturn from our solar system; however, gas giant exoplanets can be much larger than Jupiter in size.

There are further classifications in this category: for example, “Hot Jupiters” are gas giants which are very near to their stars, and hence have very high temperatures (close to solar temperature). They produce a considerable wobble in their stars, and are easier and among the first to be discovered.

  • Neptunian-like:

These exoplanets are similar in size to Neptune (or Uranus), and may have a mixture of rocky interiors with heavy metal cores and hydrogen-helium dominated atmospheres. These planets often have thick atmospheres preventing light from entering, hence making the study of their compositions peculiar.

In contrast to Hot Jupiters, finding “hot Neptunes” has been rather difficult and with much less success. Most of the Neptune-like planets are ‘warm’, and orbit farther from the region expected to be containing hot Neptunes. On the contrary, studies have shown that this type of exoplanets are more common to be formed and found in the outer-realm of planetary systems. There have also been recent discoveries of mini-Neptunes, which are planets smaller than Neptune but bigger than the Earth in size.

  • Super-Earth:

This is the class of planets more massive in size than the Earth, but smaller than Neptune/Uranus. Their composition can be of gas or solid rocks, or even a combination of the two, and can vary from two to ten times in mass as compared to Earth. The planets at the upper bound of this category are also referred to as “mini-Neptunes”. Even though it is one of the most common categories, with many exoplanets discovered to this day lying in this group, still not much is known to comment about the variety of the planetary compositions, and its relation to the size of the exoplanet.

Efforts have been made to study the nature of these planets, including studying their surface compositions and temperature distribution along the planet. The most studied sub-category of these planets are the “hot Super-Earths”, viz. The super-Earths orbiting very close to their host star. Temperature maps of these planets reveal extreme temperatures on two sides of these planets: the side facing the star is extremely hot while the dark side is much cooler.

  • Terrestrial:

This category includes planets similar to the Earth in size, and have a rocky composition and solid surface. Scientists are yet to discover the upper-limits of the possible size of a rocky planet. There have not been many discoveries pertaining to the presence of atmosphere, water-bodies and in the longer run, the possible existence of life on these planets.

An interesting observation is the gap in the size of these planets, known as the Fulton gap. The data for these planets shows that planets ranging in the size of 1.5 to 2 times the size of Earth, are comparatively quite rare. The proposed argument explaining this observation is that planets near the upper-bound of the range, attract large clouds of hydrogen and helium, resulting in creation of a thick atmosphere and eventually ballooning up into gaseous planets. On the other hand, smaller planets orbiting closer to their stars could possibly be cores of Neptune-like planets stripped of their atmospheres. Further explanation can be made after a deep and better understanding of the formation of these solar systems.

GENERAL CHARACTERISTICS OF EXOPLANETS

  • Surface composition and temperature:

Surface features are studied by comparing emission and reflection spectroscopy with transmission spectroscopy. The range of the radiation gives a criteria to distinguish between rocky and gaseous exoplanets: Mid-infrared spectroscopy of exoplanets may detect rocky surfaces, while near-infrared may identify magma oceans or high-temperature lavas, hydrated silica surfaces and water ice.

Measuring the intensity of the light it receives from its parent star can estimate the temperature of an exoplanet. However, there are chances of these estimates being quite inaccurate owing to their dependence on the reflective nature of the planet (albedo), which is usually unknown and other factors such as reflection due to the atmospheric elements and greenhouse effect. An alternative approach is to measure the temperature by analysing the variation in the infrared radiation emitted by the planet while orbiting around and being eclipsed by its host star.

  • Colour and brightness:

The apparent brightness of a planet depends on how far away the observer is, how reflective the planet is (albedo), and how much light the planet receives from its star, which depends on how far the planet is from the star and how bright the star is. So, a planet with a low albedo that is close to its star can appear brighter than a planet with a high albedo that is far from the star. The first discovery in regards to the colour of an exoplanet was made in 2013.

In the case of gas giants, geometric albedo generally decreases with increasing metallicity or atmospheric temperature unless there are clouds to modify this effect. Increased cloud-column depth increases the albedo at optical wavelengths, but decreases it at some infrared wavelengths. Optical albedo increases with age, because older planets have higher cloud-column depths. Optical albedo decreases with increasing mass, because higher-mass giant planets have higher surface gravities, which produces lower cloud-column depths.

  • Magnetic field:

The magnetic fields of exoplanets can be detected by their auroral radio emissions, which could enable determination of the rotation rate of the interior of an exoplanet, and is usually a more accurate way to measure exoplanet rotation than by examining the motion of clouds.

Unlike the Earth, whose magnetic field results from its liquid metallic core, super-Earths may have different compounds under high pressure, with greater viscosities and high melting temperatures, preventing the layering of the interior core.

Hot Jupiters have been observed to have a larger radius than expected, which could be caused by the interaction between the stellar wind and the planet’s magnetosphere creating an electric current through the planet that heats it up causing it to expand. The more magnetically active a star is, the greater the stellar wind and the larger the electric current leading to more heating and expansion of the planet. This theory matches the observation that stellar activity is correlated with inflated planetary radii.

  • Atmosphere:

The presence of an atmosphere is one of the first and easy to identify features of any planet. Several exoplanets have been observed to have atmospheres, most of them being either hot Jupiters or hot Neptunes that orbit very close to their star and thus have heated and extended atmospheres. Exoplanet atmospheres can be observed in two ways: First is the use of transmission photometry or spectra to detect the light that passes through a planet’s atmosphere as it transits in front of its star. The second manner to achieve this is to detect the direct emission from a planet’s atmosphere, differing the star plus planet light obtained during most of the planet’s orbit with the light of just the star when the exoplanet is behind its star (secondary eclipse).

The composition of the clouds, which make up the gas giants, depends on the temperature. With a decrease in temperature, the cloud layer sinks to a lower altitude (higher pressure). High altitude clouds often block light coming from deeper layers of the atmosphere, hence their presence can be detected by using transmission spectroscopy in order to identify weaker absorptions than normal.

  • Plate tectonics:

There have been contradictory arguments made by two teams of researchers through their respective studies, concerning the likelihood of plate tectonics on super-Earths. While one team argues that plate tectonics on super-Earth are very likely, the other is of the opinion that such events are either occasional or stagnant. There has also been a proposition regarding the existence of continents. According to this, continents can also exist for super-Earths with less than 80 times the amount of water than that present on the Earth. For planets having larger amounts of water, the water cycle is not enough to move through the oceans, leading to an ocean planet with all the land submerged.

  • Insolation:

It is a peculiar feature observed in tidally locked planets (planet orbiting in such a way that its same side faces the star always, similar to how the Moon orbits the Earth), wherein they have their star directly shining overhead at one point always, while the opposite region receives almost no light. This leads to extreme temperatures on both sides of the planet, and is analogous to an eyeball with the hotspot enacting the pupil.

IS THERE ANY LIFE POSSIBLE ON THESE EXOPLANETS?

The field of exoplanetology has seen a remarkable growth in recent years, with many new planets being discovered and advancement in technology allowing detailed studies with more precise and informative results. However, among all these discussions, the one question which remains unanswered is the possibility of existence of life on these planets.

In order to detect the presence of life on other planets, it has to be developed at a planetary scale and also should have made strong alterations to the planetary environment which cannot be explained by the classical processes. One example of this is the presence of molecular oxygen (O2) in the Earth’s atmosphere, which is produced as a result of the process of photosynthesis. However, there may be small amounts of it present already in the atmosphere produced by non-biological means.

What is the “Habitable zone”?

The “habitable zone” is defined as the region around a star where the temperature is perfectly suited for liquid water to exist on the surface of a planet. This means that the planet is neither too close to the star for the water to evaporate, nor it is so far that the water just freezes to ice. The distance of the habitable zone varies according to the size and age of the star.

Some other prominent factors while determining the habitable zone are:

  • Atmosphere of the planet:

Certain elements in the atmosphere behaviour wise trap more heat than the others, thus affecting the net heat content on the planet surface.

  • Planet size:

Planets with larger mass have wider habitable zones because gravity reduces the water cloud column depth, hence reducing the greenhouse effect of water vapour.

  • Planetary rotational rate:

It is a deciding factor for the circulation of the atmosphere and hence the pattern of clouds: slowly rotating planets create thick clouds that reflect more and so can be habitable much closer to their star.

  • Metallicity:

Planets in habitable zones of stars with lower metallicity are more habitable for complex life on land than high metallicity stars due to the stellar spectrum of high metallicity stars being less likely to cause the formation of ozone, hence allowing more UV exposure of the surface.

RECENT DISCOVERIES

As discussed earlier, the field of exoplanets is quite unexplored and has only recently been a frequent subject of discussion. With the potentially enormous possibilities of exploration that the subject has to offer, it has kept astronomers and researchers quite engaged, resulting in tremendous discoveries and spectacular results. While most of these results confirm conventional approaches, there are yet some which pose a challenge and have even questioned our understanding of the universe and its ways. Some recently, intriguing discoveries in this sphere are listed below:

  • Gas giant with heavy elements in its atmosphere:

Gas giants are often found to be containing hydrogen and helium dominant atmospheres. It has been well established that the higher the mass and more massive the size of a planet, the higher the proportion of lighter gases in its atmosphere, with most discoveries conforming to this notion. However, the recent discovery about the atmosphere of the gas giant Smertrios, by the James Webb Space Telescope, has been found to be defying this hypothesis. With the high concentration of carbon and oxygen in its atmosphere, the composition is way off the charts for gas giants. This has led astronomers to take into consideration the possible differences and varieties in the gas giants.

  • The largest cosmic mirror:

The atmospheric composition of any planet is responsible for the amount of light reflected by the planet. For example, the thick clouds of Venus, containing large amounts of carbon dioxide, reflect around 75% of the incident sunlight. On the other hand, Earth only reflects about 30% of the incident light.

Recently, astronomers have discovered the most reflective exoplanet to be known till date. The planet, named as LTT9779 b, is located around 264 light-years from Earth and is around five times the size of the Earth. It reflects around 80% of the light that shines on it from its parent star, acting like a cosmic mirror due to being covered by thick reflective clouds of metal. It has been classified as a “hot-Neptune”, and was earlier predicted to have a low albedo owing to its high surface temperature (around 2000 degree Celsius). At such a high temperature, it was believed to have no possibility of having even an atmosphere and only left with bare rock.

The existence of this planet has led researchers to explore other theories for metal cloud formation. It is now believed that the planet acquired metal clouds owing to an oversaturation of its atmosphere with silicate and vaporised metal. However, there are still some doubts regarding this theory, and the planet has been categorised as displaying mysterious characteristics, with scientists looking for more information and better suited explanations for its behaviour.

  • The planet which was not supposed to be there:

As a star approaches the end of its life, it begins to exhaust its fuel and transforms into a red giant. This involves expansion of the star to its biggest possible size, thus swallowing everything around it upto a certain radius (depending on the expansion capacity and size of the star). However, there has been an anomaly noticed by scientists in this very respect. An exoplanet, named Planet 8 Ursae Minoris b orbits a star at some 530 light years away, which comes in the range of its host star expansion region. However, even after the star evolved as a red giant, the planet remains in its position, in a stable and nearly circular orbit, instead of being engulfed by the star. There have been two theories proposed aimed at explaining this anomaly: The planet is either a survivor of a merger between two stars, or it is a new planet formed from the debris remains after the merger.

The first proposition begins with two stars about the size of our Sun in close orbit around each other and the planet orbiting both of them. One of the stars “evolves” a bit faster than the other, going through its red giant phase, casting off its outer layers and turning into a white dwarf, while the other just reaches the red giant stage before the two collide. The remains of this merger is the red giant which has been observed, thus preventing it from growing further and sparing the orbiting planet from destruction. In the second scenario, the violent merger of the two stars ejects an abundance of dust and gas, which forms a disk around the remaining red giant serving as a raw material for a new planet to coalesce.

  • New-found planet seemingly too big for its tiny star:

    One of the recent planetary discoveries has posed a very challenging head-scratcher: the planet, named LHS 3154b, is more than 13 times the size of the Earth in mass, while its host star is just 11 percent of the Sun. This discovery questions the very established theory of planet formation, which completely denies any possibility of the formation of such a star-planet system (mass ratio wise).

The planet was discovered after astronomers noted a periodic shift in the spectrograph of the star, thus indicating the possibility of existence of a planet around it (radial velocity method of finding exoplanets). On analysing the observations, the calculations indicated that the planet is only 0.35 times the mass of its host star. According to the theory of planet formation, low-mass stars tend to form low-mass planets. This hypothesis is based on the notion that the planets form alongside their respective stars, from the same disc of materials. Hence, the possibility of existence of the calculated mass ratio, and the very less period of rotation (around 10 days) was seen as contradictory.

After proposing various models and running simulations, the team of astronomers reached the conclusion that the explanation for such observation could be due to the very high amount of dust in the protoplanetary disk. However, there is still no rigid understanding as to why this dust is not seen in all the observations. This planet is being considered as an extreme example of the “missing mass” trend spotted in some earlier observations as well.

REFERENCES:

  • https://en.wikipedia.org/wiki/Exoplanet
  • https://exoplanets.nasa.gov/alien-worlds/ways-to-find-a-planet/
  • https://exoplanets.nasa.gov/what-is-an-exoplanet/planet-types/overview/
  • https://en.wikipedia.org/wiki/Extraterrestrial_atmosphere#Exoplanets
  • https://exoplanets.nasa.gov/search-for-life/habitable-zone/
  • https://www.space.com/webb-telescope-hot-jupiter-exoplanet-atmosphere
  • https://www.aanda.org/articles/aa/full_html/2023/07/aa46117-23/aa46117-23.html
  • https://sputnikglobe.com/20230711/astronomers-discover-shiniest-exoplanet-with-reflective-metal-clouds-1111806386.html
  • https://science.nasa.gov/universe/exoplanets/discovery-alert-the-planet-that-shouldnt-be-there/
  • https://www.astronomy.com/science/the-planet-lhs-3154-b-seems-way-too-big-for-its-tiny-star/
  • https://www.popularmechanics.com/space/deep-space/g40958720/exoplanets-in-the-milky-way/