More than 25 centuries ago a new way of understanding reality and the human relationship with it came into being: science as a method, with philosophy as its substratum. In fact, Anaximander, a pupil of Thales of Miletus, the “father” of philosophy, had already speculated about the possibility of multiple worlds when he wrote the first treatise on this discipline in the 6th century B.C. Anaxagoras, Leucippus and Democritus maintained this tradition in the following centuries, until it was completely vanquished by a geocentric[1] and anthropomorphic vision that was to hold sway for almost two thousand years. Dismantling this view has been an enormous task that still continues today.
Among the many critical milestones in this process was the discovery of the first planet outside the Solar System orbiting around a star with similar characteristics to those of the Sun. Named 51 Peg b, it was discovered by Michael Mayor and Didier Queloz in 1995 using the radial velocity method. According to this method, the planet’s existence, which is undetectable by conventional methods, is shown indirectly by the tiny movement of the star around the center of mass common to the planet-star system. Since then, several hundred planetary systems have been discovered, confirmed or validated [2] using various techniques. Currently, the most productive method is the use of planetary transits, which are analogous to solar eclipses. The planet, by hiding a small fraction of the star, produces a slight variation in the light reaching us from it, and the periodicity and other characteristics of this change allow us to infer the planet’s presence. The undisputed king in this field is the NASA satellite, Kepler, launched in March 2009, which in its five years in operation has permitted several thousand candidates to be identified. However, additional investigations are needed, since Kepler is also very effective in discovering double stars that hide each other regularly, known as eclipsing binaries. Sometimes the effect observed can be similar to that caused by planets, meaning that a binary star may mimic a planetary transit. Therefore, confirmation by other method continues to be necessary.
Kepler-447: river valleys versus glacier valleys or binary stars versus planets
In what can be considered the veritable treasure chest produced by the Kepler satellite, we have found a real gem: a solar analog hosting a hot Jupiter that orbits close to the star. The planet’s existence has been confirmed both by analysis of the light curve, provided by the satellite and due to partial occultations, and by radial velocity data acquired by CAFE, the Spanish instrument installed in the German-Spanish Astronomical Center at Calar Alto. What is unique about this system is the variation in the eclipses perceived in different sweeps when passing in front of the stellar disc These irregularities may indicate the presence of a third body, also invisible to the methods normally used. How is this possible?
The star named Kepler-447 is included in this list of candidates, although it was assumed that it probably did not host a planet and was actually an eclipsing binary. Its spectral type, G8V, corresponds to a cooler solar analog. Its light curve shows variations every 7.8 days. In principle, these could be caused by a companion star of similar characteristics, owing to the peculiarities of the reduction in the light curve during the eclipse, which is V-shaped, while a conventional transit is U-shaped. In other words, it could be a system formed by two mutually eclipsing stars. The difference between the light-curve minima in “U” and in “V” have some parallels with Earth’s geology and valleys. Excavated by glaciers or by rivers flowing on a plain, their beginnings can be identified by their form: “U” for glaciers and “V” for rivers. Thus, morphology denotes origins. Something similar occurs with planets and eclipsing stars.
Along with Jorge Lillo-Box and other collaborators, and as part of the former’s thesis, we have confirmed the planetary nature of Kepler-447 b by acquiring data with the CAFÉ instrument. To this end we have measured variations in radial velocity as the planet orbits the star, which is the same method that was used to find 51 Peg b, the first planet outside the Solar System.
A double enigma: how can the invisible be detected?
The existence of Kepler-447 b having been confirmed by the radial velocity method, the next question is, what causes the peculiar V-shape in the light during transit, characteristic of the partial eclipse of two stars, when they scarcely block the light from the one that is behind? The answer is simple: they are partial transits, in which only about 20% of the planet covers a tiny fraction of the star’s disk. However, the most important and unique piece of the puzzle is still missing. The behavior of Kepler-447’s light curve is unique, and has not been observed until now. The V-shape decreases are not always the same; sometimes the dimming of the star is less than expected. In other words, the depth of the “V” is not always the same. This has several possible explanations. One of the most probable is the presence of starspots, similar to the spots observed on the Sun [3], above all at times of maximum activity. Stains, with different temperature from the rest of the stellar surface and lower brightness, would be partially hidden by the planet, and the result would be a non-periodic variations in the light curve during transit. However, in the case of Kepler-447, it would be required to be much larger than those observed in the sun, and that they were near the pole, when sun spots are usually near its equator
An alternative and much more interesting explanation would be the presence of a third body. Thus, another planet orbiting more distantly, or even a more distant satellite, can be close to the equator, perhaps like the relatively massive Io, Europa, Ganymede and Callisto, which orbit our Jupiter, could cause small changes in the relative positions of the star and planet, confirmed by the radial velocity method. Unfortunately, in the case of Kepler-447 we know we know we are dealing with an instrumental effect, but systems of this kind, with planets close to their star and producing grazing transits, are ideal for identifying other bodies. Thus, what is invisible, another planet or satellite, can be detected in a sophisticated effect seen in the light curve and its peculiar V-shape of variable depth. Similarly, the method known as TTV (transit-timing variation), which accurately measures delays and advances in the start and end of each transit, compared to the moment predicted based on previous measurements, allows us to infer the presence of a third body. Thus, both TTVs and grazing transits provide us with tools to find planets that are undetectable by other methods. Of course, it always necessary to be extremely cautious, because, in our anthropomorphic desire, we sometimes cannot avoid thinking that nature enjoys playing with us.
In any case, these methods allow complex planetary systems to be found, increasing the diversity already glimpsed at by our ancient Greek predecessors. In fact, Kepler-447 is not the first planet that our group has discovered and described. Some of the most exotic include Kepler-37 b, a planet smaller than Mercury (and, therefore, an absolute record in minimum size), more than a dozen planets of similar size to Earth (but too near to their central star to have liquid water), and Kepler-91, where a star of enormous size, at the end of its life, is about to devour the planet. As the exoplanets catalog grows [4], in great spurts, the different exoplanetary gaps are being filled in, revealing to us totally alien worlds, in configurations that not even the most unconventional minds could begin to imagine. The Universe always surprises us.
David Barrado Navascués
CAB, INTA-CSIC, European Space Astronomy Center (ESAC, Madrid)
[1] Initiated precisely by Anaxagoras, using homocentric celestial spheres, entrenched by Aristotle and disseminated as great tool for calculating the positions of stars and planets by Ptolemy during the Roman Empire.
[2] Confirmation requires additional observation methods, such as measurement of variations in the star’s radial velocity, whereas validation is a statistical process that determines the possibility of the planetary candidate being real.
[3] The Sun should only be looked at using proper eye protection, designed specifically for this task and acquired from specialized stores. Looking at our star at sunset, even if wearing sunglasses, can cause irreversible eye damage.
[4] At February 2015, 1189 have been confirmed, found in 477 multiple systems, “The Extrasolar Planets Encyclopaedia”, http://exoplanets.eu/
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