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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 (2,7, this is a bit controversial , though ***). Its life itself (3)! Joule heating (10). The radio-active elements it contains(8). And several other conditions (4). Stars that contain comparatively large amounts of heavy elements provide less favourable conditions for the emergence of complex life than metal-poor stars (9). 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 ( rocky surface)  for maintaining surface water and a breathable atmosphere  ( right atmospheric mix), which to date scientists have only been able to calibrate using observations from our own solar system, may be too limiting. If a planet is too gaseous, atmospheric pressures and temperatures will be too intense for complex molecules like DNA to be stable. In a diffuse and gassy atmosphere, it might take too long for atoms to encounter each other, react and form new molecules. And scientists 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).











10. Ofer Cohen et al, Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad206a

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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.


Paucity of phosphorus hints at precarious path for extraterrestrial life

Work by Cardiff University astronomers suggests there may be a cosmic lack of a chemical element essential to life. Dr. Jane Greaves and Dr. Phil Cigan will present their results at the European Week of Astronomy and Space Science in Liverpool.

Greaves has been searching for phosphorus in the universe, because of its link to life on Earth. If this element—with the chemical code P—is lacking in other parts of the cosmos, then it could be difficult for extra-terrestrial life to exist.

She explains: "Phosphorus is one of just six chemical elements on which Earth organisms depend, and it is crucial to the compound adenosine triphosphate (ATP), which cells use to store and transfer energy. Astronomers have just started to pay attention to the cosmic origins of phosphorus and found quite a few surprises. In particular, P is created in supernovae—the explosions of massive stars—but the amounts seen so far don't match our computer models. I wondered what the implications were for life on other planets if unpredictable amounts of P are spat out into space and later used in the construction of new planets."

The team used the UK's William Herschel Telescope, sited on La Palma in the Canary islands, to observe infrared light from phosphorus and iron in the Crab Nebula, a supernova remnant around 6500 light years away in the direction of the constellation of Taurus.

Cigan, an expert on these stellar remnants, says: "This is only the second such study of phosphorus that has been made. The first looked at the Cassiopeia A (Cas A) supernova remnant, and so we are able to compare two different stellar explosions and see if they ejected different proportions of phosphorus and iron. The first element supports life, while the second is a major part of our planet's core".

The astronomers struggled with foggy nights at the telescope, back in November 2017, and are only just starting to get scientific results from a few hours of data.

Cigan cautions "These are our preliminary results, which we extracted only in the last couple of weeks! But at least for the parts of the Crab Nebula we were able to observe so far, there seems to be much less phosphorus than in Cas A. The two explosions seem to differ from each other, perhaps because Cas A results from the explosion of a rare super-massive star. We've just asked for more telescope time to go back and check, in case we've missed some phosphorus-rich regions in the Crab Nebula."

The preliminary results suggest that material blown out into space could vary dramatically in chemical composition. Greaves remarks: "The route to carrying phosphorus into new-born planets looks rather precarious. We already think that only a few phosphorus-bearing minerals that came to the Earth—probably in meteorites—were reactive enough to get involved in making proto-biomolecules.

'If phosphorus is sourced from supernovae, and then travels across space in meteoritic rocks, I'm wondering if a young planet could find itself lacking in reactive phosphorus because of where it was born? That is, it started off near the wrong kind of supernova? In that case, life might really struggle to get started out of phosphorus-poor chemistry, on another world otherwise similar to our own."

The researchers now plan to continue their search, to establish whether other  also lack , and whether this element, so important for complex life, is rarer than we thought.

Source: Royal Astronomical Society

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


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,

Cornell University

Exoplanets: How we'll search for signs of life

A framework, called a "detectability index" which may help prioritize exoplanets that require additional study. 

 Donald M Glaser et al, Detectability of Life Using Oxygen on Pelagic Planets and Water Worlds, The Astrophysical Journal (2020). DOI: 10.3847/1538-4357/ab822d


Possible atmospheric destruction of a potentially habitable exoplanet

Astrophysicists studying a popular exoplanet in its star's habitable zone have found that electric currents in the planet's upper atmosphere could create sufficient heating to expand the atmosphere enough that it leaves the planet, likely leaving the planet uninhabitable.

Until now, planetary scientists have thought that a habitable planet needs a strong  surrounding it to act as a shield, directing ionized particles, X-rays and ultraviolet radiation in the  around and away from its atmosphere.

That's what happens on Earth, preventing dangerous radiation from reaching life on the surface, and what does not occur on Mars, which now lacks a global magnetic field, meaning any initial inhabitants of the red planet will probably need to live in underground caves and cavities for solar wind protection.

The new research, by Ofer Cohen of the Lowell Center for Space Science and Technology at the University of Massachusetts Lowell and colleagues, published in The Astrophysical Journal, examined whether electric currents generate in the ionosphere of the exoplanet Trappist-1e would lead to enough heating and expansion of the atmosphere that it might dissipate away from the planet's gravity and be lost to space.

TRAPPIST-1e is a cool M-dwarf star in the constellation Aquarius about 41 light-years from Earth. Its , which has seven observed exoplanets, is the most closely studied system outside our own solar system.

Three of these planets are in the star's habitable zone, with surface temperatures where liquid water could exist. Because M-dwarfs, which comprise about 70% of stars in the universe, are cooler than our sun, these zones are much closer to these stars.

Trappist-1e, an exoplanet discovered in 2017, orbits just 0.028 AU from its star (where 1 AU is the average distance from the sun to Earth; Mercury orbits at about 0.4 AU). Rocky and Earth-like, its average density is only 2% larger than Earth's, and its surface gravity 82%. What's more, it has a equilibrium temperature of 246 Kelvin, just 9 K below Earth's.

These properties make Trappist-1e one of the most interesting of all exoplanets discovered to date. But does it have an atmosphere? Because it is located much closer to its star, atmospheric stripping by stellar winds should be much stronger than, say, Mercury's, which has no atmosphere.

Earlier work showed that stellar winds from Trappist-1 could potentially strip a hydrogen-rich atmosphere from its exoplanets by photoevaporation, but modeling complexity means these planets could have a host of atmospheric environments.

But another potential stripping mechanism is when external charged stellar winds impact the ionized upper atmosphere. In earlier work Cohen and others found that when the conductance and impedance of each are similar in magnitude, the three Trappist exoplanets e, f and g, could experience  (DC) resistive heating of up to 1 watt per square-meter, 1% of the incoming solar irradiance and 5 to 15 times the stellar energy from extreme-. Such "Joule heating" could potentially strip the atmosphere from any of these planets. (On Earth, Joule heating is about 0.01 W/m2.)

Now Cohen and colleagues have modeled a second phenomenon that could also impact Trappist-1 planetary atmospheres: heating due to the planet's motion itself. Alternating  (AC) will be generated in the planet's upper atmosphere as it encounters a changing stellar magnetic field as the planet orbits its star (Faraday's law of induction).

Close-in planets orbit very quickly—Trappist-1e's orbital period is just 6.1 Earth-days—and the rapid change in the background magnetic field leads to the generation of strong ionospheric currents that dissipate and create potentially very high heating, which they call voltage-driven Joule heating.

Because astronomers do not have measurements of Trappist-1's stellar wind and magnetic field, the group used validated physics-based models to calculate its energy output, its solar wind and the changing magnetic field at the Trappist-1e distance. Using reasonable estimates for the width of Trappist 1e's ionosphere, its conductance and the magnitude of the changing magnetic field, their results show that the Joule heating energy flux in the upper atmosphere of the planet would vary from 0.01 to 100 W/m2, a significant amount of heating that may be greater than that due to extreme-ultraviolet and 1 to 10% of the stellar energy flux at the planet.

They conclude that such intense values could cause a strong atmospheric escape and "could lead to a rapid loss of the atmosphere." It means astrobiologists and others should take Joule heating into account when considering an 's habitability.

"It is likely that both mechanisms operate together in close-in exoplanets," said Cohen. "Therefore, our work (and our solar system knowledge) may suggest that exoplanets located very close to the star are likely bare planets with no atmosphere."

Cohen notes that their work has a political element, as many teams are investigating the atmospheres of Trappist-1 planets. The James Webb Space Telescope (JWST) has already started to observe this system's planetary atmospheres (finding none), and there are plans to do more. "This may be a bit of a waste of resources if there is no atmosphere to study," said Cohen.

 Ofer Cohen et al, Heating of the Atmospheres of Short-orbit Exoplanets by Their Rapid Orbital Motion through an Extreme Space Environment, The Astrophysical Journal (2024). DOI: 10.3847/1538-4357/ad206a

***Plate tectonics not required for the emergence of life
Scientists have taken a journey back in time to unlock the mysteries of Earth’s early history, using tiny mineral crystals called zircons to study plate tectonics billions of years ago. The research sheds light on the conditions that existed in early Earth, revealing a complex interplay between Earth’s crust, core, and the emergence of life.

Plate tectonics allows heat from Earth’s interior to escape to the surface, forming continents and other geological features necessary for life to emerge. Accordingly, there has been the assumption that plate tectonics is necessary for life.
However, a paper published in Nature examining plate tectonics from a time 3.9 billion years ago, when scientists think the first traces of life appeared on Earth shows a different view. The researchers found that mobile plate tectonics was not occurring during this time. Instead, they discovered, Earth was releasing heat through what is known as a stagnant lid regime. The results indicate that although plate tectonics is a key factor for sustaining life on Earth, it is not a requirement for life to originate on a terrestrial-like planet.
Researchers found that there wasn’t plate tectonics when life is first thought to originate, and that there wasn’t plate tectonics for hundreds of millions of years after. The data suggests that when scientists are looking for exoplanets that harbor life, the planets do not necessarily need to have plate tectonics.
Stagnant lid tectonics: an alternative to plate tectonics

Earth is a heat engine, and plate tectonics is ultimately the release of heat from Earth. But stagnant lid tectonics—which results in cracks in Earth’s surface—are another means allowing heat to escape from the interior of the planet to form continents and other geological features.

Plate tectonics involves the horizontal movement and interaction of large plates on Earth’s surface. Tarduno and his colleagues report that, on average, plates from the last 600 million years have moved at least 8,500 kilometers (5280 miles) in latitude. In contrast, stagnant lid tectonics describes how the outermost layer of Earth behaves like a stagnant lid, without active horizontal plate motion. Instead, the outer layer remains in place while the interior of the planet cools. Large plumes of molten material originating in Earth’s deep interior can cause the outer layer to crack. Stagnant lid tectonics is not as effective as plate tectonics at releasing heat from Earth’s mantle, but it can still lead to the formation of continents.

Early Earth was not a planet where everything was dead on the surface. Things were still happening on Earth’s surface; This new research indicates they just weren’t happening through plate tectonics. Researchers had at least enough geochemical cycling provided by the stagnant lid processes to produce conditions suitable for the origin of life.

While Earth is the only known planet to experience plate tectonics, other planets, such as Venus, experience stagnant lid tectonics.
People have tended to think that stagnant lid tectonics would not build a habitable planet because of what is happening on Venus. Venus is not a very nice place to live: it has a crushing carbon dioxide atmosphere and sulfuric acid clouds. This is because heat is not being removed effectively from the planet’s surface.

Without plate tectonics, Earth may have met a similar fate. While the researchers hint that plate tectonics may have started on Earth soon after 3.4 billion years, the geology community is divided on a specific date.

Scientists think plate tectonics, in the long run, is important for removing heat, generating the magnetic field, and keeping things habitable on our planet. But, in the beginning, and a billion years after, the data indicates that we didn’t need plate tectonics.

John A. Tarduno, Rory D. Cottrell, Richard K. Bono, Nicole Rayner, William J. Davis, Tinghong Zhou, Francis Nimmo, Axel Hofmann, Jaganmoy Jodder, Mauricio Ibañez-Mejia, Michael K. Watkeys, Hirokuni Oda, Gautam Mitra. Hadaean to Palaeoarchaean stagnant-lid tectonics revealed by zircon magnetismNature, 2023; 618 (7965): 531 DOI: 10.1038/s41586-023-06024-5


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