We No Longer Harbour Any Doubt That We Are Not Alone Even in Our Own

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 most 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 Kepler space telescope 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 Hubble space telescope 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.

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