Discovery methods to find new worlds

Why are exoplanets hardest to detect by conventional methods? Why do large institutes like NASA or ESA rely on Artists to create Exoplanet images based on Real data from space missions and Satellites? Today, we'll discuss such topics and find out that Exoplanet discoveries are nearly impossible with optical Telescopes. Hello and welcome to today's post. Without wasting any time, let's begin.

Introduction:

An exoplanet or Extra-solar planet is just a planet that orbits other stars, Clear? The main difficulty is their size and distance. To see a star, you don't need anything; we can clearly see the Average stars from 100s of light-years away. Stars are actually giant light sources, while planets are pretty small compared to stars. On top of that, they don't have their light; they just reflect incoming light from a nearby self-illuminating source. Just for example, our Sun can fit 1300000 Earths or 1000 Jupiters, so they won't be visible even from just a light year through naked eye or conventional telescopes.

The last time, the first predicted planet was Neptune, which was discovered by the German astronomer Johann Galle on the suggestions and coordinates calculated by Urbain Le Verrier and John Couch Adams on 23rd September 1846. But nowadays, discovering exoplanets has become only a game of predictions and calculations through Solar models and predefined knowledge. Today, we will see some of those methods that are used to find or estimate planets around other stars.

Before we start, there are a few things to consider. We might think other methods like Spectroscopy, which is the Investigation of Light, hence its source, but in this case, the light is reflected and can't reach more than a few Astronomical Units (AU). Plus, this object is very small.

So we need to rely on clues and minor signs. Like, Radial velocity, Transit, microlensing, Astrometry, Imaging, or Timing methods. The methods of Astrometry and Timing are derived from Radial velocity. Today, we'll try to understand the mechanics behind them and their related Satellites.

Radial Velocity:

This is quite an interesting method to find signs of planets around a star. In our school textbooks, we were taught that our Earth and other solar system planets move around the Sun. This is partially true. In actuality, the planet or any orbiting body moves around the Center of mass of the parent body.

Any rotating feats like rotating a key chain around a finger must be aligned with the position of the center of mass (ring's center), which will be located inside your finger; otherwise, failure happens. This is somehow different in the astronomical contexts, because planets are small and less massive, but they are capable enough to move their parent a little bit. It is done by mutual gravitation; hence the Star or parent body tends to orbit in a very small circular path, and often you won't gonna notice it. You may wonder if we say our Earth is being pushed or pulled by the moon, and yes, it is.

The Animation of Slight Stellar Wobble

That's how planets or moons orbit their parent bodies; they also tend to push their parent bodies and parent orbit in a very brief circular path. It is dependent on the planet or moon's size and mass. Since we can't measure the light coming from an extrasolar planet or exoplanet from such a large distance, we can measure its star and its position. Studies show that All Stars with planetary systems often wobble in their position, otherwise it was only considered true for the Sun and its planets. Let's see how tiny these wobbles are that our instruments can detect.

Let's say somewhere but not everywhere, A star has an Earth-sized planet and nothing else except Minor Bodies like asteroids, comets, and debris rocks. So, this planet will make its star wobble in only 450 Km Radius, at a speed of 9 cm per second. Quite uninteresting, yeah?

The most interesting fact is, can our Telescopic legends and Space probes detect this almost-nothing-to-star thing? The answer is No. Whether it's HARPS, Kepler, or any other current equipment. None of those will be able to detect such a tiny vibration despite its immense properties. You might fathom how difficult this method is for finding an Earth twin in several light-years away.

what about large planets like Jupiter or Saturn, do they move the Sun as well? Absolutely, they do. Now, let's drop our imagination of the Sun as a giant ball of plasma and other planets as orbiting balls around it. In reality, each of the solar system's major entities, like planets, moons, and rings are giant complex or system, including our sun, which interacts with each other.

So, the Sun also orbits its center of mass or wobbles since the Sun is a very large object. The largest effect is generated by Jupiter's Gravity, which is only 1000th of the Solar mass, yet so strong. It can make the sun wobble 738,840 Km Radius with a speed of 12 meters a second. Now this large influence can easily be detected by current technology. That's why Scientists find mostly Jupiter-like planets from several light-years away, but it doesn't mean there is no Earth-sized rocky planet; they are hidden due to all those effects that we're discussing here. We just need to improve our specs.

The instrument that detects such wobbles is usually called the Doppler Spectrograph, which shows the curves in a sinusoidal waveform of the light source; it doesn't detect planets but shows only clues. One of the Famous Doppler spectrographs is the High Accuracy Radial Velocity Planet Searcher, or HARPS, in short. Detect 1 m/s Radial velocity or wobble from a light source that has an appropriate distance. The others are SOPHIE, CHIRON, CARMNES, and EXPRES. There are almost 1131 Exoplanets have been discovered thus far through this method.

One more thing, the enormous Radius of wobble can form a Barycentric orbit if two masses are enough to move each other. For example, Alpha Centauri consists of two stars, actually, it's 3, but two of them are in the barycenter. The massive Star α Cen A wobbles within a Radius of 1.6 billion Km with a speed of 3.99 Km/s due to Star B. Whereas B does it in 1.9 billion km Radius and with almost 4.7 Km/s speed due to Star A. This won't be seen as wobbling; instead, one will call it that they are orbiting each other. They don't shake, instead they are in a common Barycenter. If our Jupiter were more massive, A least an M Type Star, our Sun would have a binary partner.

Astrometry:

This method concerns the discovery of the Exoplanet based on the Star's position or movement in a defined reference system. Since Star is not free from the effects of planets, especially massive ones like Jupiter. The expectation and speculation for celestial bodies have been used for time immemorial, whether it's Chinese, Hindu, Mayan, Egyptian, Babylonian, or Roman culture.

Every culture had its own method to plot astronomical objects. We have some evidence of Star tracking from ancient ages, which involved plotting the stars based on several reference points or fixed coordinates, constellations, Buildings, Structures, seasons, specific events, etc. We don't know why they were tracking the Stars, and they showed such a very deep interest at that time that they constructed Several ancient observatories, Pyramids, tombs, temples, and other various instrumental buildings that can be used to measure and locate specific Stars.

Astrometry is not a new method, but it was a part of ancient history; the only differences are the global coordination and the usage of highly advanced instruments. So, let's first see how Stars are plotted in the current system. By the way, there are many reference points, but we'll see the most commonly used System called the Horizontal Coordinate and Equatorial Coordinate system. The Horizontal is basic, while the Equatorial method is an advanced and precise reference system.

The Horizontal system is Real-time and depends on your location on Earth. Suppose you see the Orion belt stars horizontally from your location at night, then it might be seen diagonal or vertical, or with any other orientations from elsewhere. Scientists work solely and cooperate with others from another corner of the world. It requires a fixed and globally valid system, which is the Equatorial system. In further Sub-sections, we'll explore both methods.

I) Horizontal system:

Suppose our local sky is a giant dome that has the same horizontal and vertical lines as on real-world Maps. Now we have lines, let's mark some lines to memorize them well. Imagine we have a compass, and North is at 0 unless you don't move your head up or down, in front of your nose tip, just look at a common straight line toward the 0 Degree. Your right Ear will point 90 Degrees or East, your back will point 180 degrees, and finally your left ear will face 270 degrees, and last 360 or 0 is the same forward direction. Likewise, 12 O'clock, 3, 6, and 9 O'clock. In between directions can now easily be recognized.

A similar method is also used in Military Navigation. Since Stars don't appear in your front or your plane unless it's a pop, rock, or film star, or even a Star icon. We're talking about Real stars here. You know the Azimuth (the angles in clockwise Horizontal rotation) points, and now see the zenith and altitude, so let's begin.

We can directly see the top of our head, so if you look right in front of you, it will be 0 degrees, and when you look directly top of your head while you stand still and don't turn your head a little bit backward, this will denote 90 Degrees, this will be your or your location's zenith point. The in-betweens can now easily be mapped, and you are ready to plot the stars. Just don't forget to mention the observed time and location, it will be only true to your place and the recorded time.

II) Equatorial System:

This is the enhanced edition of previous reference systems, where observations won't lock any longer to location, time, or any other aspects. It's globally valid. Let's see how. In the image below, you can see, we have a planet under custody and locked inside Prison-like RGB bars. Let's know about them.

Earth-like Planet showing the Right Ascension and Declination lines

In the Equatorial system, it doesn't rely on a specific location or time; Instead, it applies on a global scale what we did in the previous sub-section. Meaning, The Red Horizontal ring you see in the middle denotes 0 degrees means it's the equator. Since it's an Earth-like exoplanet, this grid has 24 Vertical lines called Right Ascension (RA) and 180 horizontal lines called Declination angles (Dec), which capture the entire planet inside the Prison of Global Reference system. One might argue why there are 24 lines instead of 360 if it's about Earth's or the planet's full rotation?

The equatorial grid system is based on Sidereal time instead of the Earth's daily rotation. The Sidereal day lasts 23 Hours and 56 minutes. In which our Earth rotates completely 360 Degrees relative to the fixed stars, which are not a specific star. Actually, it's the point on the 0-degree imaginary line.

For our Sun, the Earth spins 361 Degrees to complete a full 24-hour cycle, and it's 4 minutes later than the Sidereal day time. Let's say Earth completes 360 degrees in 23h 56m for ease, we assume it is 24 hours. Each hour has 15 Degrees of angle, so 15 x 24 = 360. That's why Right Ascension or RA has 24 lines. This is measured in Hours, minutes, and seconds. It also reflects the fact, certain Stars with appropriate position and motion. If they rise at 8:15 PM, then the Next day they will rise at the Same time at 8:11 PM, since the Sidereal Day is 4 minutes Shorter than the standard Solar day.

Now let's talk about those Horizontal lines. The key feature is almost normal because 0 degrees is on the Equator and +90 shows the North pole and -90 denotes the South pole of this imaginary sphere, not Earth's.

Julian Epoch:

There is one vital thing: since Earth is rotating and moving, over a very long time or given time, it might change. Because the Current Equatorial system is based on the time of J2000, it was declared at midnight of 1st January 2000. Planets change their rotation axes, speeds, and orbital planes, although not noticeable by humans, which affects the precision of extremely distant objects. You know that this sphere's one rectangle or cell can contain thousands of celestial objects, and it may not change the coordinates of nearby objects, but Farther objects will start some misalignment. So, this system needs to keep updating over a given time, like there will slightly updated version in 2050 or J2050, likewise, J2100 might officially be declared in 2100 CE

It's possible that if any Star or Nebula is located at any current RA and Decs, after a very long time, it might appear on different coordinates even though this object remains still over such a long period, since Earth's equatorial alignment has been shifted. Ancient cultures were aware of this trend and had their own ideas or treatments to fix this issue.

For example, the Star Thuban or α Draconis was used to be our North Pole Star from around 3942 BC to 1793 CE, where Egyptian culture symbolized it as the North Pole star. Currently, The Star α Ursae Minoris is the North Pole star, which reached near Earth's North Pole in about 500 CE, and it will remain in the Pole till 3000 CE. Facts like these can challenge our current understandings. So, they must be updated over time.

Now this is clear, RA and Dec provide coordinates to locate celestial objects, any minor change from predicted vs observed coordinates can be a key to the discovery of an Exoplanet, since the Host Star's changed motion can be affected by its planets. This method has given us only 20 Exoplanets thus far. Note that NASA lists only 5 of them as discovered by Astrometric methods due to rigid rules. Since this method requires Extreme sensitivity of instruments. GAIA and HIPPARCOS Missions have found Jupiter-like Planets that can easily drift the Star and make changes in our Coordinate systems.

Timing method:

If you think for a while, you'll find that those methods require a very long time of observation, almost 10 or 20 years sometimes, or even decades, because stars move or change their position very slowly. Is there any method that can tell us shifts in some minutes or seconds, as if real-time watching? Yes, in which scientists can observe in a few days or weeks instead of long waiting. This involves the help of some variable things at steady rates, like: Pulsars, Variable Stars, and periodic eclipsing binaries. We'll discuss how helpful they are in estimating Exoplanet clues.

The Neutron Star Showing features of a Pulsar and Gravitational lensing

A pulsar is a kind of Neutron Star that emits radio wavelengths at a specific frequency and period. They are so accurate that even after millions of years, they almost don't lose consistency. Pulsars are small objects that form when a Star between 8 to 27 Solar masses becomes a Supernova, and its outer layers are scattered in space while its core shrinks up to 15-20 Km due to gravity. This compressed core is called a Neutron Star. Since celestial objects spin on their axes, if they get compressed, their spin gets faster. In the case of a Neutron Star, they usually complete one full rotation in a few milliseconds. Our Earth has an average rotation of 86400 Seconds or 24 hours, the Sun completes one full rotation in almost 25 days, while a common Neutron Star does it in just 1 to a few hundred milliseconds. Imagine how fast they are, just a 20 Km sphere rotates in such a short time.

Neutron Stars have extremely powerful Magnetic fields, and they create Relativistic jets if they are in accretion disks. Since they rotate so fast, and if their jets are pointing toward us, it will make it flashing effect or change their brightness due to one after another coming ahead of us like a lighthouse's light. This kind of neutron star with a pulsating light source is called a Pulsar. They are used as a Metronome or a Stopwatch. They are one of the most accurate clocks. This helps us to match the perceived time in space.

The Common Space Scene containing a Pulsar

Let's talk about the Pulsar planets, which forms when Turbulence in the Accretion disk slows down and particles start to stick with each other. Since The Neutron Stars have powerful radiation and Magnetic fields. The probability of Planets is very low. Their formation can be interrupted by Pulsar emissions, while we have discovered some planets around pulsars, which tells us how our current understandings get crippled by the Universe's wonders.

If there is any planet orbiting a pulsar, it will cause changes in the Continuous Radio emissions of the pulsar; these minute anomalies will not go unnoticed by our current instruments. The same for eclipsing binaries and Variable Stars' light pulsations, which change in seconds, minutes, or hours, which are really shorter periods as compared to methods like Transit, Radial Velocity, or Astrometry.

Since planets around pulsars are a very rare thing, they form by a little bit of luck and appropriate conditions, like the slowing down of accretion disks, which stick particles together further, which can create planets.

The Pulsar PSR B1257+12 has 3 planets, while PSR B1620+26, PSR J1719-1438 have one planet; these are confirmed planets. While PSR B0943+10, PSR B0329+54 and PSR B0144+59 are suspected or not confirmed yet. There are around 4 confirmed planets if we exclude suspicious ones. Likewise, we have some 4 planets around Variable Stars and approximately 8 planets around eclipsing binary systems, including suspicious objects.

The total number of planets discovered using this method is around 66 planets including Variable Stars, Eclipsing binaries etc. The TESS, TTV, HST, KST, GBT, and SST are key instruments/telescopes for timing methods, both directly and indirectly.

Microlensing:

Let's now see the lensing method. Lensing in astronomy means the Bending of light by a strong gravitational field. This light comes from the Background, and the lens forming object is foreground, it's the exact position between Observer-lens-Background light source (or Sources).

Suppose you hold a magnifying glass and look around through it while holding it a little bit far from your eyes, and you see that the image gets distorted, looks upside down, magnified, or even blurred. No, this effect doesn't use the lens forming object as a magnifying lens and zoom the light source (let's say a suspected star) to find a hidden exoplanet.

Instead, this method uses a formed lens made of a combination of Star+Planet, which can bend the light coming from thousands of light-years away and make a distorted image of the background from thousands of light-years away. You might have seen the gravitational light bending or lensing around the images of Neutron Stars, Dark matter halos, Galaxies, Black holes, or Quasars. Note that their lensing effect is pronounced and easily observable in certain conditions.

Since Planets are not so massive as them, they show this effect very rarely and sometimes our eyes can't detect their lensing effect is minute, hence it is called Microlensing.

To understand it, look at the image of Pulsar from the previous section and look at this image below, which suggests a several Jupiter masses planet around this star. It's an artist's impression; in reality, it's a number and estimation game on computers. We created this image to understand how it might be,

Artist concept of a massive Exoplanet showing Microlensing

There are almost 250 Exoplanets have been found using this method. The well-known Involved instruments are OGLE, MOA, KMTnet, HST and the GAIA space observatory.

Imaging:

In the case of Exoplanets, with the current scene, Imaging means getting an image of the exoplanet as a pixel or point-like shape. Whether it's Visible or Infrared light. This is one of the hardest jobs to execute. Because Stars are a billion times brighter than planets, most of the exoplanets are so close to their host stars meaning they'll show tiny angular separation and almost undetectable using current conventional telescopes, even with the sizes of 10m or 20m.

Let's see the Angular separation and why it's important. If you look in the night sky and see the Alnitak, Alnilam, or Mintaka Stars (The 3 stars in the Orion constellation that appear to align in one straight line). Alnitak is a triple star system, Alnilam is a single Star and Mintaka is a 5-star system, which appear as single stars from Earth due to Distance. If you look with a sufficiently large telescope or go near those stars, their individual stars will start to appear separately. This is the effect of Angular Separation.

Now, imagine we have a distant star that has an exoplanet almost the size of Jupiter. It won't appear even with larger telescopes or in visible light because the planet is very small compared to its star. If it's orbiting so close around under 1 AU, then other methods will work. If it is 5-10 AU away from the star, then we could use the imaging method because it will give us a better angular separation.

What if Star is shiny and the planet can be hidden inside its intense light? We can use a block or veil to hide the star-like solar eclipses, where the moon comes right in front of the Sun, and due to the Moon and Sun sharing the same Angular size in the Earth's sky, the Moon can almost cover the Sun's photosphere and reveal the corona, in which we see the diffused filamentous luminous structure.

But how do we do it to other stars? It's simple, we can use a block or a veil inside the telescope to simulate an eclipse. This method is called Coronagraphy and the instrument inside the telescope is called the Coronagraph. Of course, we can't use a common circular patch; instead, it is made of certain materials with different qualities. The overall goal is to reduce the sheer stellar light so that the planet can be visible. Infrared light has been most successful in this method, Since Gas giant planets radiate it mostly. 

The Concept of Direct imaging of a Star system containing 3 planet

This method is about to create noisy, low-resolution or sometimes blurry images of the Planets around stars, not for taking HD photos. Due to current limitations, we can only pay attention on Low resolution photographs. Below you can see an example of a generated image using the Imaging method of a planetary system. One more thing, sometimes, Star's light can create such artifacts, so they have to be careful with such events and capture a true pixelated image of the planet.

Almost 100 planets have been discovered using this method. The famous Star HR 8799 Has 4 planets. in older studies there was 3 planets but now there are 4.

Transit:

Let's talk about the final yet famous method of Exoplanet detection, Transit is basically an eclipse that can happen to any light source by an obstacle. Since the Sun and Moon have similar Angular sizes in Earth's sky, the Moon can easily block the Sun and make the eclipse simple. As we know, this is impossible if the light source is distant and the obstacle is so small. Here, we call the host star a light source and the obstacle its orbiting planet.

Imagine you are on the tip of Pluto's mountain, looking at the sun and you know that Jupiter is gonna eclipse the sun, meaning it will come between the sun and your line of sight, it will create a slight blink on the sunlight, and you will never know whether it blocked or not. Because, we don't have that ability, but our satellites do. So they will easily detect Jupiter blocking the sun. So it's also easier for advanced technologies to detect such light blockings if the star is bright enough and the object is sufficiently large to create a massive interruption in stellar light.

This is like detecting a housefly flying around a giant Spotlight located in Europe from North America. Of course, it doesn't come in visual animation as CCTV footage or what you see on our post. they come in numbers and parameters, and by checking them, scientists estimate clues of such exoplanets.

Some specialized Satellites/observatories/missions in this method are: TRESS, TESS, Kepler, PLATO, TRAPPIST, COROT etc. Some of them are still operating or have been discontinued. These satellites detects the dips in suspected light sources, which can take some days to a few years. If there is one Star and its planet completes an orbit in 2 years, it will show one light anomaly once per two years if we exclude other light-blocking effects like Interstellar Medium regions sometimes can change the Light behavior, Dust clouds or Relative motion etc.

However, not every planet can be discovered using this method. Let's see its condition. Some of the conditions we've talked about. If The Planetary system is edge-on, not face-on, then it will show transit.

To understand it, look at the image below. The left side shows the Face-on angle where the planetary orbits have been shown, and it will not create any transit, while the right side will because at least one of those 3 planets will block the star's light coming toward us sooner or later in the course of their orbit.

The concept of Face-on and Edge-on perspectives

This will need perfect synchronization of our position, light, and this planetary system; otherwise, it won't transit. However, this is pretty common since we live in a spiral galaxy where Stars mostly orbit in a planet-like flat plane, so transiting events are not as rare as you think. We've discovered around 4340 planets, where Kepler alone found 2784 planets and TESS is also continuing to increase these numbers every year.

Current limitations:

We know that exoplanet discovery methods are constantly evolving, yet they need a lot of improvement. like the 80s-90s times, this is still a number game and heavy mathematical analysis. If any star is tremendously Bright or massive, like B or O Stars, our satellites won't be able to detect such minor irregularities. That's one of the main reasons why the exoplanets are very rare around massive stars.

Most of the planet-hosting Stars are F to M-type Stars, which are less massive and less bright enough to show radial velocity, Astrometric perturbations, and other observable signatures by their planets. Other is most of the Exoplanets are noted as coordinates of such anomalies, never directly seen like Uranus or Neptune planets.

That's why their star's coming light is the only way to detect their whispers for now. Scientists are planning to make 100 meters large telescopes, powerful space telescopes and cutting-edge technologies. The current generation of technology can easily detect Jupiter or Neptune-like large planets, while Kepler has also been successful in finding hundreds of Earth-like planets, but that's not enough. Wait and watch until technology advances.

Thanks for reading this post and coming here.

Have a nice day and night.