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How Many Planets Are There? (3/3)

It seems that the answer to this question is rather elusive. About a week ago we said that after the fall of Pluto from the planetary pantheon, the answer was 8. Then we saw some more can be found planets orbiting strange and inhospitable objects, such as neutron stars, but we only found a dozen by this method .

But as I said, knowing the number of stars in the universe (a number) accounts simply do not exist. Indeed, we are already seeing more and more planets, which convinces us that are not so strange.

And how can we discover these extra solar planets? Because it’s one of the most active branches in current astrophysics, detection methods have multiplied in recent years. The following image, published by M. Perryman in 2000, shows one of the most commonly used methods:

 
The method of radial velocities is based on the oscillatory motion. This wobble of the star will be more noticeable the larger the mass of the planet in relation to the mass of the star, so this method is more efficient to detect large planets, several Jupiter masses. It is called radial velocity because thanks to the Doppler effect, one can measure when the star moves away or approaches us, measuring the frequency shift of a spectral line. That is, when the star approaches us (because the planet is pulling it toward us in its movement) the spectral line will run a little to the blue, while if the star is moving away, the line will run until the Red. By measuring the frequency shift and having an estimate of the mass of the star, one can limit the size of the planet and some orbital parameters. This all sounds we known, because the method is similar to how stars were discovered pulsars .

The other method mentioned is the method of transits, which measure the changes in brightness in a star as the planet orbits it passes in front of it (relative to us). These small “eclipses” produce variations of brightness tiny but measurable. As in the previous case, the high – mass planets tend to obscure more than smaller planets star, so the first will be easier to detect.

Today they have reached more than 750 planets detected by making use of all these methods, and the census can go at every day in the catalog of the Extrasolar Planets Encyclopaedia .

There is currently a great competition in terms observatories and instruments capable of detecting the highest number of extrasolar planets. Examples are space missions COROT  (the European Space Agency ) and Kepler (from NASA ) using the transit method. On Earth, we have as an example the project SuperWASP (transits) in which participates actively IAC , and the instrument HARPS (radial velocities) of the  European Southern Observatory  (ESO), which runs on the 3.6m telescope at La Silla Observatory in Chile.

Finally, I cite the recent press release from the ESO, very relevant as to the question at hand:

Our new observations with HARPS mean that about 40% of all red dwarf stars have a super-Earth orbiting in their habitable zone, a zone that allows the existence of liquid water on the surface of the planet.” says Xavier Bonfils (IPAG, Observatoire des Sciences of the Universe of Grenoble, France), who leads the team. “Because red dwarfs are so common – there are about 160 billion in the Milky Way – this leads us to the conclusion that there are tens of billions of such planets in our galaxy alone.

There is still much to learn and to say about this fascinating topic so much interest. But what is certain is that in the coming years will go knowing more about planets, the composition and conditions of their atmospheres, and if they meet the requirements to support life.

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.

How many planets are there? (1/3)

This question automatically activates our hippocampus and transports us to our childhood where one day we learned Image of Saturn captured by Voyager 2the planets of the solar system: “Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.” All this only altered by the grand finale of Pluto, our subsequently banished distant planet. The 9 planets of the Solar System  where it appeared, may have included the image of Saturn in false color (see afterwards that it was a composition of green ultraviolet light and violet) taken in 1981 by the space probe NASA ‘s Voyager 2 (see photo right).

Since the discovery the last and most timid on the list (Pluto) was back in 1930, one wonders how planets are discovered. An important detail of the discovery of Pluto it is that, as usual in astronomy, it was not by chance. The search for Pluto was a result of the discrepancy between the observed position of Neptune and the theoretical position of accurate astronomical calculations. In fact, some similar calculations eight decades ago by John Couch Adams and Urbain Le Verrier independently, led to the discovery of Neptune, which also seemed somehow alter the orbit of Uranus.

The discoverer of Pluto, who changed the long standing list of planets, was Clyde Tombaugh. From the Lowell Observatory in Arizona he discovered the slow movement of the icy dwarf and ran out giving it the name of the god of the underworld.

He used this device, known as blink comparator, which is a completely obsolete in our modern times. If someone wanted to see live a living relic of this device can be found in the museum of Lowell Observatory (pity there is not one in the collection of the National Geographic Institute).
Another story of Pluto is that of a girl of 11 years. Specifically Venetia Burney (pictured), a student at Oxford at that time who proposed Pluto as possible name for the newly discovered star.

I can not end this first entry without mentioning the fall of Pluto of Olympus in 2006. The IAU decided that Pluto should no longer be a planet because it was too different from the others. In particular it is too small and far away. Possibly the straw that broke the camel’s back was the discovery in 2005 of the trans – Neptunian object Eris, which is actually larger than Pluto. This forced the IAU in 2006 redefined the term planet, to prevent about 25 objects similar to Pluto from being classed as planets. I guess kids today can thank the IAU to spare them memorize more than 30 planets!

In conclusion, Pluto since 2006, along with 4 other colleagues (Ceres, Eris, Makemake and Haumea) have been classed as dwarf planets, and as a result, the number of planets that hypothetical children present should memorize (if educators do not believe too stressful) is eight. So (for now) the answer to the title of the entry is eight .

In my next post we will go a little beyond Pluto to see how planets of other stars are discovered.

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