Quanto Magazine

How many planets are there? (2/3)

We spoke a few days ago what one immediately thinks for “planet” when you hear that word. Mercury, Venus … etc, until a few years ago, it was certainly no more than ten. In recent decades there have been dozens of space missions to different planets, comets and satellites. The results of these explorations have largely been available to the general public that have seen detailed images of Mars sent by the probes Viking and Mars Pathfinder.

But even though the exploration of the solar system still has a long way to go, the idea of worlds (and life) around other stars is surely back to the dawn of human imagination. Interestingly the first two planets detected are not found orbiting a quiet star (such as our beloved Sun), but in a strange place
where no one would have thought to find such a thing: the pulsar PSR 1257 +12 that is in the Virgo constellation about 2,000 light years from us. It came to fame in 1992 for harboring two planets of a size about twice that of our Earth. The discoverers of such a finding were the Polish Alexander Wolszczan and Canadian Dale Frail.

The pulsars (see the artistic recreation of one on the right) are tiny neutron stars that spin at breakneck speeds (specifically PSR B1257 + 12 has a diameter of about 30 km and a rotation period of 6 milliseconds, or that is, rotates about 160 times per second!). A curiosity is that both the discovery of this pulsar and observations that revealed the presence of these exoplanets were made at the Arecibo radio telescope in Puerto Rico. Antenna we have all seen in the movie “Contact” based on the novel by Carl Sagan. Interestingly this telescope today provides data project SETI @ home that seeks to detect radio emissions by extraterrestrial civilizations.

The method to detect these planets around pulsars is quite simple to understand. Pulsars have a period of extremely regular rotation, so the radio broadcast we received with a radio telescope has a fairly well-defined period. One example is the following video showing an auditory representation of electromagnetic pulses that are detected in some famous pulsars:

Now imagine a small planet revolving around one of these pulsars. This pulls gravity on the planet in the same way that the planet pulls on the pulsar, or in other words, both bodies will revolve around the center of mass system common. In the case of system PSR B1257 + 12 and the largest of its planets (called B as the second in distance) the center of mass is about 700 km from the pulsar (remember that its diameter is about 30 km). To calculate this figure just we need to know that the planet B has an estimated mass 4.3 times the mass of Earth and is orbiting at about 0.36 astronomical units pulsar).

From our reference system on Earth, the pulsar will have a very small oscillation around its center of mass, which will add to the intrinsic rotation of the neutron star. Thus the periodic pulse emitted will be modulated in turn by this oscillation another almost imperceptible.

The residue left after subtracting the period of the pulsar can determine the axis of the orbit of the object that produces the disturbance (the supposed planet), as well as having an upper limit on its mass.

Of course all this is complicated when instead of having a single planet we have a planetary system with several components. Each planet then exerts a little tug on the pulsar and slightly modulate its intrinsic pulse. So it becomes even more complicated demodulation. This is the case of PSR B1257 + 12 in which 4 planets have been detected in different sizes and orbital distances.

The advantage of this method is that it is quite accurate because the pulsar signals can be digitized with enough time to be picked up by radio telescopes resolution. In addition to being the natural clocks so precise pulsar is easy to detect tiny anomalies in the period due to relatively small planets, even smaller than Earth. No other method to discover planets as small.

The disadvantage is that there are not many known pulsars which severely limits the potential planets to discover. In fact, today, less than a dozen extra solar planets have been discovered using this technique. In addition, another point that takes some of the interest the matter is that all these planets would not harbor life almost certainly. Pulsars are neutron stars that remain as residue after the recent explosion of a massive star as a supernova . Therefore any planet in the area would have to be formed at some time after the explosion and would be subjected to intense high – energy radiation that would prevent the formation of life.

I guess I still need to answer the question. We’ll save that for next week.

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