The Enigmatic Objects of Kepler-51
Anomalous Planets, or Alien Megastructures?
The world’s space enthusiasts turned their heads in 2015 when it was announced that a Sun-like star over 1,400 light years away was erratically dimming. Scrutinized through the keen eye of the Kepler Space Telescope, the star, Tabby’s Star, was determined to be exhibiting changes in luminosity that almost certainly couldn’t be attributed to an orbiting planet, nor the star’s natural fluctuations. In the wake of the discovery, many scientific theories were fronted to explain the curious dips in brightness, which implied the presence of something far larger and less periodic than a planet. Amidst proposals of planetary collisions, evaporating comets, and asymmetrical dust rings, one exciting theory seized the attention of the public—an eclipsing alien megastructure. An explosion of public interest ensued as the luminosity dips were imagined to be the eclipses of an unfinished Dyson Sphere or Ring World, a possibility that the scientific data couldn’t initially rule out. Though the changes in brightness were later determined to be caused by anything but large solid objects, the alien megastructure theory was taken seriously by many researchers, inciting the creation of numerous scientific publications. If nothing else, the excitement surrounding Tabby’s star in 2015 taught us that we may be seeing more than just planets out there…
Of course, the Kepler Space Telescope made a whole slew of profound discoveries between its launch in 2009 and its deactivation in 2018. Using the data from Kepler’s observations, astronomers discovered gas giant cores stripped of their atmospheres and denser than iron, planets larger than Jupiter in orbits tighter than Mercury’s, and even small rocky worlds that bore a striking resemblance to our own pale blue dot. Though many of these discoveries have challenged and modified our historical models of planetary formation, some continue to elude any form of scientific explanation whatsoever. A poster child of such a discovery is the planetary system of Kepler-51, a Sun-like star located about 2,600 light years from Earth. The system consists of three ultra-low density planets, dubbed “super puffs”, which have masses on the order of just a few Earths, but radii rivalling Jupiter’s. Assuming spherical geometries, the three planets would average densities equivalent to cotton candy — 30 times less dense than Jupiter. Of the 4,377 confirmed exoplanets, none exist quite like them.
Unlike hot Jupiters, whose gaseous radii are bloated by heat from their nearby suns, the planets of Kepler-51 are relatively distant and cool. Boasting equilibrium temperatures that average between 350 and 550 Kelvin, the planets’ atmospheres may feel like a warm day in Tahiti, but are a far cry from what is normally required to expand them to such extremes. Another explanation is that perhaps the planets are still in a state of formation, the apparent magnitude of their eclipses amplified by thick clouds of accreting planetary material. Though the half-billion year old system is relatively young, protoplanetary disks usually collapse and coalesce into planets on the order of tens of millions of years, implying that the system should be long completed with that phase of its evolution. Every other super puff planet identified has fallen into one of these two rationales, and none are as tenuous as the planets of Kepler-51. While statistics may allow for a few erroneously low density planets to fall outside of these explanations, for there to be three all in one system almost certainly displaces any chance of error. The Kepler-51 objects are real, so what are they?
In order for astronomers to determine an exoplanet’s density, two variables are needed: mass and volume. Two inherently different methods are used to discern these properties; mass — by means of the “radial velocity method”, and volume — assumed by radius, determined via the “transit method”. In the radial velocity method, exoplanets are discovered by observing how a host star wobbles, and then applying gravitational equations to determine what mass an orbiting object would require to explain the star’s motions. The radial velocity method is robust in determining the existence of exoplanets, but is sometimes unreliable in determining the true mass of exoplanets since the amplitude of the star’s wobbles also depends on the angle that the system is being viewed from the Earth, which often times isn’t know. In the transit method, a star’s luminosity is observed for an extended period of time for periodic dips in brightness which may imply the presence of a transiting exoplanet. By observing what percentage of the star’s light is blocked, a very reliable estimate for the size of the object causing the eclipses can be generated. By assuming the eclipsing object is a sphere, a radius is assigned. If the transiting objects are massive enough, their masses can also be determined via the radial velocity method, and since the relative angle would be known in this case, these mass estimates would be extremely reliable. Using both factors, the density of an exoplanet can be found.
Exhibiting robust transits and being massive enough for radial velocity measurements, the objects of the Kepler-51 system seem to be gold standards of the successes of our exoplanet analyzing techniques. However there is one seldom mentioned shortcoming of the transit method — it cannot determine the true geometry of the eclipsing object. In most cases, this isn’t an issue; if a planet-sized object is determined to be eclipsing a star, it can likely be assumed to be a sphere since the gravity of all natural objects larger than ~2,000 kilometers tends to pull them into this shape. This is usually bolstered by radial velocity measurements of the star, which imply masses for the planets that suit their radii. However, this is not the case for the objects of Kepler-51, light enough to seem almost Earth-like, yet wide enough for their shadows to be mistaken for gas giants. If the planets’ erroneously low densities cannot be explained by any of our models of planetary architecture, perhaps the data demands examination in a new light. Maybe the objects are not puffy spheres at all, but are instead dense and flat, resembling the geometry of a coin. But if massive objects naturally bias towards the shape of a sphere, how would such enormous, flattened objects come to be?
The Kardashev scale is a hypothetical means of classifying advanced alien species on a scale of 0 to 3 based on energy production. On this scale, a Type 1 civilization has harnessed the energy resources of an entire planet, a Type 2 species has captured the power output of a star, and a Type 3 race controls the energy of a whole galaxy. Humanity is currently considered about a 0.7 on this scale, our energy demands escalating towards Type 1 status. If our exponential demand for energy continues, the resources on our planet alone will eventually not suffice, and we will have to look towards the Sun to satisfy our growing power needs. Theorizing how an aspiring Type 1 species might harness the power of a star, physicist Freeman Dyson imagined the construction of immense solar energy collecting megastructures in 1960, now known as Dyson spheres. Though the end goal would be a 100% efficient shell encapsulating the power output of the entire star, several other megastructures were imagined as intermediaries between the Type 1 and Type 2 stages of development. One of these megastructures, a Dyson Swarm, would consist of an array of several large, flat, solar energy collecting disks. It is possible that this is what we are observing in the mysterious Kepler-51 system.
When this model is applied to the data, it fits shockingly well. Given a density of steel, the three disks would yield thicknesses of around 200–500 kilometers which, compared to their radii, is the same thinness aspect ratio as a music record. The orbital distances of the three objects are also almost perfectly spaced, and are in a near 1:2:3 orbital resonance. Though this geometry is not unheard of for natural planets, it would also be an optimal configuration for power collection and transmittal if the objects were artificial. The three objects are also the perfect distance from their sun for power collection — not too close for there to be stringent thermal constraints, but close enough for maximum irradiance. This is bolstered by the fact that the objects are in order from smallest to largest with distance, since a larger radius object would be needed farther away from the host’s light. Finally, the combined masses of the Kepler-51 objects (13.7 Earths) is almost identical to the mass of solid material in our own Solar System (~16 Earths, including the gas giant cores). This may imply that an advanced species found a way to manipulate the system’s solid matter into the disks we observe today. All this, in tandem with the fact that Kepler-51 is a very Sun-like star, make it an enticing candidate for alien megastructures.
Since the Kepler-51 system is so young, it is unlikely that complex life would have had time to evolve within it. This, however, does not preclude the possibility that the orbiting objects are indeed artificial. A species vying to harness the power of an entire star may not want to destroy their own planetary system to do so, and may instead seek out a young, healthy star with lots of remaining energy output such as Kepler-51. Still, as I have discussed in previous articles, constructing any of the Dyson megastructures would be no simple task. In this case, a material with a compressive strength of ~200–300 Gigapascals would be needed; about 150 times that of steel and 4 times that of diamond. In addition, the megastructures would require extremely advanced asteroid defense systems, orbital control systems, and wireless power transmittal systems to succeed. Though these feats seem insurmountable to us, they may yet be child’s play for a species capable of interstellar travel.
In the near future, a spectroscopic analysis of the eclipsing Kepler-51 objects may verify whether these enigmatic bodies are indeed super puff planets ignoring the laws of planetary science, or perhaps something even more astonishing. Even if the objects of Kepler-51 turn out to have a grounded explanation disbarring extraterrestrial intervention, it is observations such as this which keep our minds sharp to the possibility that we may not be alone in this vast sea of emptiness. As our technology advances and our observations of the cosmos increase, so will the number of anomalies and outliers in our database of the final frontier. It will then be up to our brightest minds to determine how such objects exist, or, in cases where they can’t determine, to discover what these objects truly are. The Tabby’s star’s and Kepler-51’s of our search for intelligent life may come and go as quickly and fruitlessly as the next, but they continue to keep our minds fixated on the ultimate question — are we alone in the universe?
