In The Search For Type-I Civilizations (Astronomy)

Summary:

Extrapolating the idea of SpaceX’s Starlink constellation, Osmanav assumed that an alien society with index K = 0.7 (our civilization) will reach Type-I in 1000 years, which is enough to build a planetary megastructure for collecting the required material.

He analysed launching from the point of view energy costs and it has been shown that energy required to construct a megastructure is small compared to the total energy received from the star.

He have also shown that the construction will be visible in the IR spectrum, which might be detected by VLTI instruments up to the distance 260 light years, with ∼ 10³ Solar-type stars.

He also emphasised that the spectral variability method might be an efficient tool to detect either orbital rotation of the solid megastructure or internal motions of small satellites.

He estimated the possibility to detect the radio emission operating on the Hydrogen atoms frequency and found that for reasonable parameters FAST can detect such radio sources from relatively large distances.


Article:

Last year on May 24 the SpaceX launched the first set of 60 Starlink satellites and up to now the total number is approximately 400 aiming to reach at least 12000 at the end of a decade program announced by Elon Musk. Such a huge number of satellites, distributed over almost the whole surface of the Earth might be considered as the first prototype of a possible megastructure around the Earth, which in principal, might be visible from the cosmos. Similarly, one may search for techno-signatures of alien civilizations.

The discovery of the Tabby’s star and ”Oumuamua” have provoked the revival of the search for extraterrestrial intelligence (SETI). The idea to search for techno-signatures of advanced alien societies have been proposed by Dyson in 1960. Assuming that a civilization is advanced enough to build a megastructure around a host star to consume its whole energy, Dyson has concluded that such a huge (having the length-scale of the order of one AU) spherical construction – Dyson sphere (DS) – should be visible in the infrared (IR) spectrum. Civilizations harnessing the host star’s total energy belong to the Type-II societies according to the classification by Kardashev. Type-I civilization is harnessing the total energy coming from the sun to the Earth. Our society is consuming less than the mentioned energy, therefore, an index, K = log10 (P) /10 − 0.6, introduced by Shklovskii & Sagan in 1966 for Earthlings is 0.7, where P denotes the average harnessed power in Watts. In the framework of the same classification Type-III is the alien high tech society which is able to use the total energy of the host galaxy.

It is clear that detection of Type-II and Type-III civilizations is much easier than Type-I because of the much higher total consumed energies. Therefore, a special interest in the Dysonian SETI projects deserve Type-II,III technosignatures and a series of papers are dedicated to identification of DS candidates. Dyson’s original idea has been extended to hot DSs and the megastructures around pulsars.

Despite high radiation intensity of Type-II, III technologies compared to Type-I technosignatures, the latter still can be considered seriously in the SETI context. In particular, Kuhn & Berdyugina studied effects of global warming as detectable biomarkers in Earthlike societies.

Our civilization consumes approximately 1.5 × 1020 ergs s¯1, which is less than for Type-I society, 1.7 × 1024 ergs s¯1. If one assumes 1% of an average growth rate of industry and the subsequent energy consumption, one can straightforwardly show that our civilization might reach Type-I in ∼ 1000yrs. It is quite probable that in 1000yrs the level of technology will differ from ours, likewise ours is different from the one of middle ages. Therefore, one can assume that Type-I alien society is able to cloak their planet by a sphere-like (or ring-like) structure to harness the total energy emitted from their host star toward the planet.

Now, Osmanav in his recent paper considered the possible observational characteristics of a planetary megastructure partially or completely covering an Earth-like planet located in the habitable zone.

At first he considered the question: energetically how feasible is launching material to cover an area comparable to the Earth’s surface. As an example they examined the construction designed by means of Graphene as up to now the strongest material, our civilization is able to produce. He found that the total mass of the megastructure is less than the total storage of Earth’s Carbon, 1.85× 1024 by seven orders of magnitude and thus one can find enough material to build a construction.

Another issue he addressed, is to understand time scale required for launching such a huge mass of material. Roughly speaking, the civilization can start launching material from the times when its index was equal to ours, 0.7. Then, as I have already mentioned above, to reach the Type-I level, it needs 1000 yrs. The process will be feasible if the annual rate of growth of mass launching, µ, is a small parameter. If one assumes that the first year’s launched mass equals M0, then in 1000 yrs the total mass will be given by:

M = M0 (1 + µ)1000 …. (1)

After taking into account the total mass of megastructure and the fact that up to date there are 895 satellites each with the mass 260 kg, one can straightforwardly show that µ ≃ 0.021.

Generally speaking, the megastructure might be constructed by advanced Type-I civilization from extraterrestrial resources without launch costs, but Osmanav considered the worst case: the Solar energy is stored and then utilized to launch material on the orbit.

In this case energy required to launch material on the altitude H ∼ 500km is as follows

E ≃ G×M×M_p×H / R² ⊕ ≃ 5.7κ × 1027 ergs, …. (2)

where M_p = M_E is Earth-like planet’s mass and M_E ≃ 6 × 1027 g is the Earth’s mass.

On the other hand, power from the host star toward the planet is given by

P ≃ L/4 × (R_p / r)² ≃ 1.7κ × 1024 ergs s¯1, ….. (3)

where L ≃ 3.8 × 1033 ergs s¯1 is the solar-type star’s luminosity and r ≃ 1.5 × 1013 cm is the radius of the habitable zone. Whatever the propulsion mechanism (electromagnetic launch or some other mechanisms), it is evident from Eq. (3) that the energy required to launch a thin shell around a planet can be extracted in approximately ten hours with 10% of efficiency of energy conversion. If the megastructure is a web of many satellites orbiting the planet, then the kinetic energy should be added to the aforementioned value, leading to ten times more total energy, which, energetically is quite feasible for Type-I alien societies.

Launching has been analysed from the point of view energy costs and from that we showed that the energy required to construct a megastructure is small compared to the total energy received from the star.”

— told Osmanav, author of the study

He also have shown that the construction will be visible in the IR spectrum, which might be detected by VLTI instruments up to the distance 260 light years, with ∼ 10³ Solar-type stars.

Moreover, he have emphasised that the spectral variability method might be an efficient tool to detect either orbital rotation of the solid megastructure or internal motions of small satellites.

One has to note that the SpaceX starlink satellites use radio communication. Therefore, a reasonable question might appear: is it possible to detect their radio signals by means of the China’s FAST telescope?”

— said Osmanav, author of the study

By taking into account that its system temperature is T_sys ≃ 20K and the illuminated aperture area, A ≃ 7108 cm², they concluded that the minimum spectral flux density, k ×T_sys/A, which can be distinguished from noise is of the order of 6 × 10¯24 ergs s¯1 cm¯2Hz¯1. On the other hand, by assuming that only 1% of the incident Solar energy is used for interstellar communication, one can find that if one uses the Hydrogen atom’s frequency, 1420 Hz, the FAST telescope can detect such isotropic sources up to the distances of the order of 160 pc.

Making use of simple geometry he have performed their calculations for solar-type stars with Earth-like planets and it is clear that the similar estimates can be straightforwardly performed for Type-I civilizations living in the systems with different parameters.


Reference: Zaza Osmanov, “From the SpaceX Starlink megaconstellation to the search for Type-I civilizations”, pp. 1-5, ArXiv, 2021. https://arxiv.org/abs/2103.07227


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