WHAT IF SOMETHING APPROACHES THE SOLAR SYSTEM?
Standard SETI Project regards the search for extraterrestrial intelligence directly from planets of other stars using radio and optical observational techniques coupled with multichannel spectrum analyzers and powerful algorithms potentially able to decode intelligent signals. Apart from some false alarms, during the last 50 years nothing reliable has been found yet. The SETI Project doesn't consider the possibility that other civilizations might reach us directly and that astrophysical techniques can verify if this hypothesis is true or not.
First of all we must consider the possibility that particularly advanced extraterrestrial civilizations are able to engineer their circumstellar space, at the same time being not interested to send deliberately intelligent signals out of their environment. Physicist Freeman Dyson hypothesized that they could wrap their star with an envelope called "Dyson Sphere". This envelope is more probably expected to be constituted of a swarm of many technological orbiting planetoids, which partially obscure the central star, of which we would be still able to easily detect a G-type spectrum. The envelope would partially collect the light coming from the central star and reradiate it at much lower temperature. The reradiated energy is expected to be an infrared spectrum, which is suitable for observation from space using IR telescopes such as Spitzer. Thus, in this specific case, we expect to observe an infrared excess around a solar type star. Being typically structurally stabilized while burning Hydrogen during some billion years, a solar type star cannot produce an interstellar envelope due to gas ejected from his photosphere, which would cause an infrared excess. Therefore if we observe an infrared excess that one is not of natural origin but rather a proof that circumstellar space can be engineered by some extraterrestrial technological intelligence. We would not observe only an infrared excess but also anomalous light curves caused by rotating artificial planetoids of different geometric shapes orbiting around their star. Once we are able to identify the marking of Dyson spheres - and presently some suspicion persists that star KIC-8462852 might be one of them - we can then use them as preferential targets in order to carry out a targeted search for unintentional electromagnetic signals using the standard SETI procedure in the radio and in the optical wavelength bands.
Let's now imagine that one or more artificial planetoids are detached from the Dyson sphere and vectored towards the interstellar space. In this case one or more components of a Dyson sphere would become a "Dyson ark". Why should this happen? The most logical reason would be that the central star is exhausting its nuclear fuel and is about to become a red giant, so threatening to swallow all the internal (natural) planets due to its huge expansion. This would destroy any form of life and artificial planetoids too. Therefore such a civilization, assuming that its science is able to predict the time in which their star will become a red giant, might want to send its planetoids outwards in search for new suitable stars to orbit around. Due to this it is not, in principle, impossible that one or more extraterrestrial artificial planetoids may reach our Solar System too, also due to the fact that it contains Earth, the perfect planet where to migrate (or to invade?).
In order to accomplish this task they might use several kinds of propulsion systems, such as microwave sails or matter-antimatter annihilation. This would result into a much subluminal speed and they could take thousands years to reach another star. But at the same times they would be totally autonomous worlds with many sentient beings aboard or, alternatively, with DNA samples together with nanorobots able to assemble automatically new living beings once the target planet is reached.
How would we detect such traveling planetoids? We are able to predict this. Also in this case these objects would produce a strong infrared emission due, in this specific case, to energy losses from the systems aboard and possibly radio emission too, and they would be characterized by a very high proper motion, namely their angular velocity compared to the observer would be much higher than the one of a fixed star (which typically varies only of an arcsecond in 50 years), resembling in some aspects the regime of motion of a comet.
We cannot exclude that some of the "false alarms" previously detected by standard SETI observations might be caused by the transient passage of one of these objects along the antenna lobe of a radiotelescope. In fact typical false radio alarms show a Doppler signal that is not stable as the one predicted for an extrasolar planet transmitting radio signals, but rather short lasting and never found anymore. This is due to the entrance and the exit of the Dyson ark in the radiotelescopic antenna. In reality it is possible to elaborate some strategies able to catch again the signal by moving the antenna inside an error box on the celestial sphere. Once the signal is caught again we should have at our disposal two points using which we are able to calculate the trajectory (if the speed of the ark is constant), so that we would be able to track these objects using our telescopes using a mode that is very similar to the one used to comet tracking. This operation would be highly facilitated using the new array techniques of new radiotelescopic systems such as the SKA (Square Kilometer Array). At this point we would keep on measuring the radio signal, and verify if its flux density is variable on relatively long timescales. At the same time we would immediately send information on the object’s trajectory to all the most suitable infrared and optical telescopes operating from space and from ground, which will point and measure the object using in particular CCD imaging/photometry and spectroscopy at the highest possible resolution. X-ray and Gamma-ray space telescopes might be aimed at the target as well, searching for possible high-energy bursts of radiation.
At this point we cannot exclude that the intruder may behave like a carrier, namely able to send exploration probes directly inside our atmosphere. That would be NORAD's business, of course, which should be promptly informed by astronomers. But they could be also monitored using exactly the same techniques currently used in Hessdalen-like research.
Even if so far there is no proof that all this is really happening, the possible intrusion of an alien intelligence inside our Solar System can be ascertained de facto using astronomy-like techniques.
And, yes... I am a compulsive alien hunter, as I am searching for all the possible ways that can satisfy the scientific method. The one proposed here is not the only one, of course.