# Exoplanets

Exploring the universe

## The mystery with Tabby's star

The Soviet astronomer Nikolai Kardashev proposed in 1964 a method that is measuring a civilization's technological advancement based on how much energy they are able to consume.
This is called the Kardashev scale and has three categories.

1. A type I civilization or a planetary civilization are able to harness and control all energy available on their planet. With estimated energy use of size 10^16 watt
2. A type II civilization or a stellar civilization is able to harness and control energy at a scale of its solar system. With estimated energy use of size 10^26 watt
3. A type III civilization or a galactic civilization is able to harness and control energy at a scale of its host galaxy. With estimated energy use of size 10^36 watt

The Kardashev's scale could be calculated with the formula suggested by the astronomer Carl Sagan

$$K=\frac{\log_{10} P-6}{10}$$

where P is the energy use in watts. The current energy usage of the human civilization is 18*10^16 and has a K value of $$0.72=\frac{\log_{10} 18*10^{12}-6}{10}$$

and we will probably reach Type I civilization status in 100-200 years. Perhaps we will reach type II in a few thousands of years.

One hypothetical technology advancement to reach a type II civilization would be to build a megastructure that encapsulates the entire star and captures most of its power output. This is called a Dyson sphere and was first suggested by Freeman Dyson in his paper "Search for Artificial Stellar Sources of Infrared Radiation" from 1960, but the concept was previously described in a science fiction novel Star Maker by Olaf Stapledon in 1937. The megastructure could be technically possible by using solar power satellites or space habitats orbiting around the star in a so-called Dyson swarm. Such construction would dim the light of its host star and SETI has adopted this assumption in their search for extraterrestrial life.

Image credit: NASA/JPL-Caltech

In September 2015 citizen scientists from the Planet Hunters project discovered unusual light dimming around the star KIC 8462852 when they were analyzing data collected by the Kepler space telescope. The star is of F-type with 1.4 the mass of our sun and is located in the constellation Cygnus 1470 light-years from earth. The star got the name "Tabby's Star" after the American astronomer Tabetha S. Boyajian, the lead author of the scientific paper describing the unusual light fluctuations. The fluctuations can not be caused by a planet, because the dimming is irregular and it has been measured to vary between 22 percent and 5 percent. Analyses of old data have shown the brightness of the star has also been dimmed by 14 percent since 1890. The star is too old for the planet-forming event still would occur. The SETI Institute used the Allen Telescope Array to look for radio signals from possible intelligent aliens. In fall 2015 the news that SETI was investigating an alien megastructure got viral and took the internet by storm. The light fluctuations have been studied in 2017 and now in 2019 by TESS. The star is still a SETI target as a natural phenomenon causing the dimming is still not fully explained.

As not all wavelengths are equally blocked the most likely explanation is that the dimming is caused by dust and not aliens. Inward migration of Jovians could make their moons escape their gravity and become a so-called ploonets. Numerical simulations have shown that when these ploonets melting they spread a dust cloud around the star and that could explain the light curve of Tabby star.
A study with 21 candidates star with similar light curves was published this year that suggests that this natural event might be common, but more investigations are needed to confirm if these candidates really are tabby like.

Dyson Tabby star