We no longer harbour any doubt that we are not alone even in our own galaxy Milky Way, leave aside the whole universe, which, incidentally, is just one of an infinite number of universes according to many cosmologists. The number of planets discovered outside our solar system stood at about one thousand at the end of 2013. Over three thousand five hundred more were awaiting confirmation.Yet these numbers, unthinkable twenty years ago, sink to insignificance with what we are hearing from astronomers nowadays. Backed by a torrent of data which an array of ever more sensitive instruments provide, they estimate the number of Earth-like planets in Milky Way with orbits within the “habitable zones” suitable for life, in tens of billions, not to mention as numerous “Hot Jupiters” getting roasted in orbits almost skimming the surfaces of their stars, or ice worlds orbiting way, way out. Astronomers make these exciting discoveries, by using imaginative methods. Let’s have a look at the main ones. RADIAL VELOCITY METHOD:

Two gravitationally bound bodies in space orbit each other around a common center of gravity. That is, a planet does not revolve around a star; the star and its planet revolve around each other. But because the star is far more massive than the planet or planets orbiting it, this common center of gravity lies somewhere within the star’s radius. That means, star, too, follows an orbit ─ be it very small ─ around that common center of gravity within itself. The tangible effect of this emerges as a periodic “wobble” in the movement of star. Hence, if the wobble is in our line of sight, the star slightly approaches and moves away from the observer with regular intervals. This movement causes tiny fluctuations in the spectrum of the light coming fom the star, due to a process called “Doppler shift”. When the star is coming toward us, the spectrum of its light shifts to shorter wavelengths, towards blue light. And as it moves away, the shift is towards the longer, red wavelengths. Astronomers monitoring the movement of likely planet-harboring stars with extremely sensitive devices, confirm the presence of the orbiting planet from these miniscule periodic shifts in the starlight’s spectrum

The advanced of these devices dubbed “spectrometers”, are sensitive enough to detect velocity changes of one meter per second. The method, also known as “Doppler spectrometry”, is one of the most successful in the ongoing planet hunt. Before the began smashing records with the “transit method” it employed, most of the extrasolar planets were discovered with this method.

But although the method is independent of distance, it allows the monitoring of stars with a maximum distance of 160 light years from the sun (one light year roughly equals 10 trillion kilometers) since it requires a far stronger signal than the background noise for necessary precision. This method is particularly suitable for the detection of Jupiter-size giants in close orbits to their stars (so called “hot Jupiters”) but the detection of planets orbiting at great distances require years of monitoring. Planets on orbits with wider angles to our line of sight produce smaller wobbles and hence are harder to detect.

The mass of a star can be inferred from the spectrum of the light it emits from its surface. For, the color of its light is a function of its surface temperature (see, “How stars are classified” in Wide Angle section) Theoretical models for stellar formation and evolution permit the calculation of star’s mass, age and chemical composition from its temperature. And once the star mass is known, the magnitude of the wobble enables the determination of the planet’s mass. A problem with this method is that it can only give a minimum mass for the planet. The true mass can be 20 percent above that limit. If the orbit plane is tilted close to right angle to our line of sight, the inferred mass is closer to the true value. When the method is used together with the transit method to confirm detections, the mass of the confirmed planet can be more precisely calculated. ASTROMETRY METHOD:

Yes, but what if the star’s wobble is not radial, but lateral? If, in other words, the planet’s orbit plane is perpendicular to our line of sight? Or, in even more simple terms, if we are viewing the putative planet’s orbital plane from above? Well, the circular or elliptical orbital motion of the star around the common center of gravity can give a clue about the presence or absence of a planet. But since the orbital radius will be very small (center of the gravity being within the star,) it will be hard to detect. In fact, planet discoveries reportedly made in 1950s and 60s with this method, were later proven false.

But the method can be used in a different way. What the astronomers have to do is finding a nearby fixed “reference star” to the rear of star with the suspected planet. It is important that this reference star should be relatively stationary, for some stars have high “proper motions” and their positions in the sky may change during long years of planet search.

The wobble which gives away the presence of a star can be detected if the target star is seen periodically moving toward and away from the fixed star. Yet changes in the positions of the monitored stars are so tiny that adequately precise measurements cannot be made even with the most advanced telescopes on Earth. But in 2002, astronomers using the employed the astrometric method to determine the parameters of a planet discovered earlier around the star Gliese 876.

Despite its shortcomings, the upside of the astrometric method is its particular suitability for the detection of planets on distant orbits. This feature makes it a complementary tool for other methods normally more sensitive to closer orbits. Downside, however, is years or even decades of monitoring required for the detection of planets on orbits distant enough to allow the use of astrometric methods, since they take very long times to complete their orbits.

TRANSIT METHOD:

A planet passing in front of a star under observation causes a tiny dip in the intensity of its light. Analysis of such periodic dips, recorded with sensitive measurements, can reveal the presence of a planet or planets orbiting it. The advantage of this method compared to radial velocity and astrometry is that reveals the size (radius) of the planet. This is a key parameter, for when taken together with the mass determined with the radial velocity method, the density of the planet can be calculated, which, in turn, yields information about its physical makeup (whether it’s a rocky planet, a gas-giant, or a water world with a global ocean). The method also provides data on gases in the atmosphere of the planet as well as their ratios. As the planet traverses its star, gases in its atmosphere absorb some of the spectral lines in the starlight. And the locations and thicknesses of these absorption lines identify the gases and their ratios in the planetary atmosphere. The presence of a planetary atmosphere (and thereby that of the planet itself) can be gleaned from the measured polarization of the starlight passing through or being reflected off the planet’s atmosphere.

Another advantage the method provides is the ability to measure the radiation emitted by the planet. If during the secondary eclipse (when the planet moves behind its star) , the photometric intensity (luminosity) is subtracted from the value for the period before or after the secondary eclipse, the remainder will be the planet’s share in the total. And once that parameter is known, the surface temperature of the planet and even the possible signs of cloud formation can be figured out. In fact, surface temperatures of two planets were determined this way by two separate groups using the Spitzer space telescope in 2005. Planet TrES-1 was found to have a surface temperature of 790 ⁰C, while that of HD 209458b was an even more scorching 860 ⁰C. The COROT spacecraft lofted by the French Space Agency to make sensitive observations with the aim of finding a few earth-mass planets discovered two such worlds. Yet the undisputed king of the transit method was the Kepler spacecraft NASA had launched in 2009. Before stuck gyroscopes ended its main mission of planet hunting, it monitored 150.000 stars at the same time every 30 minutes for four years to flag rocky Earth alikes. The perused data for the first three years yielded 3538 planet candidates, 167 of which were confirmed. Kepler data showed that most of the planets in the Milky Way were small planets with masses similar to Earth’s and among them, the number of those orbiting their stars in “habitable zones” at right distances where temperatures permitted the existence of liquid water essential for life as we know it, could add up to tens of billions.

The method, however, has two serious drawbacks: First of all, for the discovery of a planet with this method, the orbital path of the planet has to be on the same plane with the observer’s line of sight. In other words, the observer has to view the orbital plane from edge on, an alignment with an extremely low probability. The probability of observing a planet as it transits its star on its equatorial plane is mathematically expressed as the ratio of the star’s radius to the radius of the planet’s orbit. Of the planets with close orbits, only 10 percent are observed as transiting their stars on their equatorial planes. This ratio goes further down for the planets with distant orbits. The probability of a sun-like star being observed as it is transited by an Earth-like planet at a distance of one astronomic unit (AU = Sun-Earth distance, or 150 million kilometers) is 0.47 percent. Still, the number of planets discovered in transit surveys monitoring thousands or even hundreds of thousands of stars at the same time may exceed the number of those bagged with radial velocity method. But there is another problem here: Buradaysa bir başka sorun var: It is not possible to identify the host star of the discovered planet. Still another problem is the unreliability of the method, necessitating subsequent analysis with the radial velocity method for confirmation of discoveries.

PULSAR CHRONOMETER METHOD

Pulsar is a special kind of neutron stars. The latter are products of supernovas which put a spectacular end to the short lives of giant stars. When no longer able to generate the energy to counter the weight of outer layers, the core of the massive star collapses on itself and the ensuing shock wave tears apart the star and catapults the outer layers to space with an explosion visible from billions of light years away. The core of about 1.5 solar mass is squeezed so tight that it becomes a sphere of 12-20 kilometers diameter ─ about that of a medium-sized town. The collapse so much whips up the original spin of the collapsing star that the neutron star completes a revolution in the order of milliseconds. The spin has a regularity exceeding those of most precise chronometers. Another feature of the neutron stars is the immense power of their magnetic fields, which can be trillions, or even quadrillions of times more powerful than that of the Earth. They emit powerful beams of radio waves from their magnetic poles.

Such radio-emitting neutron stars are called pulsars. Pulsing behavior is a result of the often- misaligned magnetic and spin axes of the star (just as the case with geographic and magnetic poles of the Earth). The misalignment causes the magnatic poles to draw circles around the spin axis as the neutron star rotates. And when a point on that circle crosses the line of sight of a powerful radio telescope on Earth, radio pulses are received from that point in exceedingly regular intervals (in the order of seconds for most pulsars or even thousandths of a second for some which are called millisecond pulsars). Since the intervals between the pulses are extremely regular, anomalies in these enable the observers to trace the pulsar’s movements. If they have planets circling them, as is the case with normal stars and their planets, pulsars and their planets, too, orbit a common center of gravity. And variations in the times of pulses give away the presence and masses of the planets.

The method is so sensitive that it permits the detection of planets even with masses a tenth of the Earth’s. It can also capture the gravitational interactions within a planetary system. In 1992, astronomers Aleksandr Wolszczan and Dale Frailbir gezegen sistemi içindeki karşılıklı kütleçekim etkileşmelerini de belirleyebiliyor. 1992 yılında Aleksander Wolszczan ve Dale Frail made use of this method to lure out the planets around pulsar PSR 1257+12.

However, since pulsars are relatively rare objects, It is doubtful that high numbers of planets can be detected with this method. Even supposing that they could, the emergence of “life as we know it” is impossible because of the extremely high-energy particles and radiation spewed out by their parent pulsars.

MİKROMERCEKLENME YÖNTEMİ:

Let’s suppose we are observing a star to detect a putative planet: One of the stars in the background of the target star is also in our line of sight. Suddenly we see the background star brighten and after a while return to its former luminosity. Now we can begin to search for the planet in earnest, for what we have seen is a microlensing event. The path of the light coming from the background star was warped by the gravity of the star in our line of sight. According to Einstein’s theory of general relativity, what we sense as gravity is actually an effect of the curvature of space-time. Any object which has a mass bends the space-time. The photons of light coming from the star behind follow the curvature of this warped space and change direction. That is, a greater number of photons start coming in our direction, or, in other words, they are focused. Thus, we see an increase in the brightness of the star behind.

But things are not all that simple. Microlensing is a variant of the gravitational lensing phenomenon, which has emerged as the outcome of one of Einstein’s famous thought experiments and has been verified by astronomical observations numerous times. When the “gravitational lens” in between is as massive as a galaxy or even a cluster of galaxies, the “source” which we cannot directly observe as it is hidden behind, is naturally a source as big as another galaxy. And since the intervening “lens” bends the light coming from the source, multiple images of the source in the form of elongated (and brightened) segments of a loop, differing according to small imperfections in the alignment of observer, lens and source, appaear around the lens. Because the alignment of the observer on the Earth, lensing galaxy or cluster and the source galaxy does not change much over thousands or even millions of years, these multiple images of the source galaxy remain in place, and detailed analyses of the images enable precise calculations of the distance, mass and shape of the galaxy lying behind.

But difficulties arise when the lens is a small astronomical object such as a star or even a planet. For one, in the observer-lens-source alignment, the source has to be behind and slightly above the lens. The lensing effect in such an alignment produces only two images of the source focused as arches and the distance between these two arches are so small that even the most powerful telescopes on earth fail to resolve them into distinct shapes. In the end, two separate shapes are perceived as a single, superimposed image. And the name “microlensing” derives from the fact that separation between the two arches is too small to be imaged.

Another problem is the brevity of the microlensing event, lasting a few days or weeks because the Earth and the source and the lensing star are in motion relative to each other.

If the foreground (lensing) star has a planet too, the gravitational field of the planet makes a detectable contribution to the lensing effect, thereby giving away its presence. But since the probability of such an alignment is extremely low, to catch a meaningful number of planets using this method requires the simultaneous and continuous observation of a large number of distant stars . Therefore, forming collaborations among themselves like OGLE (Optical Gravitational Lensing Experiment), MOA (Microlensing Observations in Astrophysics) and PLANET (Probing Lensing Anomalies NETwork), astronomers have turned their observing instruments to the dense central bulge of the Milky Way and its satellites, the Small and Large Magellanic Clouds in the southern celestial hemisphere. The surveys have yielded at least two yet-unconfirmed and two confirmed planetary candidates.

An obvious problem of the method is that since the fortituous alignment cannot be repeated, the microlensing is a one-off event, not leaving adequate time for extensive Planet OGLE-003-BLG-235/MOA discovered by the OGLE collaboration in 2003 with the studies. And since the microlensing method, and its star which could be discovered two years later. detected planets are kiloparsecs away, the find cannot be confirmed with other methods. (A parsec is a unit used for long distances in astronomy corresponding to 3.26 light years. A kiloparsec is 1000 parsecs, or 3260 light years).

CORONOGRAPHY:

Because the light a star emits is thousands or even millions of times brighter than the light reflected off a planet, normally the light planet reflects cannot be seen. But if the starlight is blocked by an opaque mask installed in telescopes, called a coronograph, the feeble light from nearby planets may come into view. Especially if the planet is large (Its radius has to be much bigger than that of Jupiter), is far fromits star and if it is young. Youth causes the planet to be hot and radiate strongly in infrared.

One of the most dramatic discoveries made with the coronography method was the detection of the planet orbiting Fomalhaut, one of the brightest stars of southern skies, 25 light years away. Despite the masking, the light of the A-class star, more massive and hotter than the sun, is seen as leaking out from the perimeter of the coronograph as spokes.

A triple planet solar system announced on November 13, 2008, one of the discoveries mentioned in the caption was detected through observations made with Keck and Gemini telescopes, among the largest on Earth. Hubble’s discovery of Fomalhaut’s planet of three Jupiter masses was made public the same day. Both systems are surropnded by disks reminiscent of the Kuiper belt in our solar system. Finally, with the detection of the planet orbiting Beta Pictoris, this method, too, took its place among the more promising of the planet hunting instruments. CIRCUMSTELLAR DISKS

Many stars have disks of space dust surrounding them. These are also called debris disks. What makes them visible is their absorption of the starlight, which then they re-emit in the infrared. Although the total mass of these dust particles is less than Earth’s mass, they outshine the stars they orbit in infrared wavelengths because of the vastness of their combined surface area. Those disks, which the Hubble and Spitzer space telescopes can pick up, have been found around 15 percent of the stars lying relatively close to Sun and having similar masses. The dust in these disks is thought is believed to be relics of collisions between comets and asteroids. Since the radiation pressure from the star should have blown the dust to space in a relatively short time, the continuing existence of dust disks leads to the conclusion that the dust is continuously re-created through collisions and attests to the existence of such star formation leftovers surrounding the central star. For instance, the dust disk around the star tau Ceti, is seen as the sign of existence of belt of rocky and icy bodies and comets, reminiscent of the Kuiper belt outside Neptun’s orbit in our Solar system, only ten times thicker.

Signs were seen pointing to the presence of comets around Beta Pictoris, a young star 20 million years old at most. These dust disks are thought to be relics of collisons between leftovers of star formation like asteroids and comets.

Meanwhile, certain features observed in the structures of dust disks, could signify the persence of planet-size objects. Holes observed at centers of some disks, show that the disks are circular. The empty region is thought to have been created by a planet which has swept away the dust lying between itself and the star. Some other disks display bulges which could have been formed by gravitational pull of a planet. Both these features are seen around the star epsilon Eridani, indicating that besides an inner planet discovered earlier with the radial velocity method, another is orbiting at a distance of 40 astronomical units.

THE ROAD AHEAD

SPACE OBSERVATORIES:

Measurements made in space yield more precise results both because the atmosphere’s distorting effects are avoided, and the observation equipment can make use of infrared wavelengths blocked by the atmosphere. Beyond the detection of rocky Earth-alikes, astronomers aim to study the makeup of the atmospheres of such worlds and search for signs of life with observations from the space.

The Kepler space telescope which NASA launched in March 2009 simultaneously monitored 150.000 stars at constellation Cygnus before losing its planet hunting ability due to malfunctioning gyroscopes. NASA is now seeking ways of assigning the spacecraft to alternative tasks which do not require a fixed orientation.

NASA is planning to launch Kepler’s successor, the Transiting Survey Satellite (TESS) in 2017.

The new spacecraft, which will be set on an elliptical orbit around the the Earth and the moon, will survey the whole sky, unlike Kepler which could only track an area the size of one four-hundredths of the space.

To be able to do that, TESS will not point continuously to the same spot, it will change its orientation every month. The interesting candidates it finds will be inspected from the Earth by the existing and next generation telescopes with gigantic light harvesting mirrors of 20-to-30 meters.

Satellite clusters that would jointly seek planets using interferometry such as the , proected by the European Space Agency (ESA) and NASA’s Terrestrial Planet Finder were later shelved due to technological hurdles and prohibitive costs entailed.

ECLIPSING BINARY PHOTOMETRY

If the stars of a binary system orbiting a common center of gravity are positioned as eclipsing each other in our line of sight, the system is called an “eclipsing binary”. When the star with the brighter surface is eclipsed, even partly, by the disk of the companion, the phase with the lowest total luminosity is dubbed the “primary eclipse”. And when the brighter star masks a part of its companion half an orbital tour later, a “secondary eclipse” is observed.

These phases of minimum luminosity, occur with a regularity rivaling the precision of a pulsar, the only difference being periodic dips in the intensity of light instead of bright light pulses. In case a planet is orbiting the binary, the stars of the system will also orbit the common center of gravity with the planet and there will be a periodic displacement in the times of lowest luminosity (the minimum will be delayed, will be on time, will be ahead of time, and then will lag again etc.) Following these periodic time shifts are seen as the most reliable method of detecting planets orbiting binary stars.

REFLECTION/EMISSION MODULATIONS

Kepler was to shoulder the task in this method; but now this, too, will seemingly be assumed by TESS. Besides its main target of terrestrial planets, the spacecraft will also keep an eye on the ligh reflecting off the giant planets in very close orbits around their stars. Since such a planet will have phases ranging from total blackness to total reflectivity just like the phases of the Moon. Periodic variations, however miniscule, in the reflected starlight will herald the presence of a planet. Because the phases of the reflective light will be independent of the inclination of the orbital plane. Astronomers believe the method could provide information as to the makeup of the planet’s atmosphere.

POLARIMETRY

The light emitted by the star is not polarized. That is, it oscillates in random directions. But when the light is reflected off the atmosphere of a planet, light waves interact with molecules in the atmosphere and get polarized. Measurements can be made with extreme precision on the light given off together by the star and its planet ─ the latter’s share being one part in a million. Devices used for measuring the polarization, called polarimeters, have the ability of selecting the polarized light and rejecting the unpolarized light (of the star). Although such collaborations as ZIMPOL/CHEOPS and PLANETPOL are searching for with polarimeters, no planet has been detected with this method so far.

Raşit Gürdilek 17 December 2013 REFERENCES: Methods of Detecting Extrasolar Planets, Wikipedia .org

TAGS: “Extrasolar planet”, exoplanet, radial velocity, astrometry, transit method, transit