An alien civilization wishing to go and do something in another solar system has to carry along with it enough baggage to allow the task to be completed. Since baggage is very expensive in interstellar flight, perhaps trying to consider what is involved might clarify the possibility of some tasks.
As noted in the post on cost-benefit analysis of interstellar travel, the only goal which makes sense is colonization. There are varieties of tasks included under this heading, and this blog has divided them into three categories. One, A1, is colonization to bring the species of the alien planet to other planets, and establish a colony of the same species there. The species may be improved through genetic engineering, but it is the same species that evolved out of primordial soup. A second, A2, is on a mission to spread life where there is none, and to bridge the Great Filters, which stop life from evolving into intelligent life. There are a large varieties of dead-ends that a planet where life originated, called solo planets here, can get into, and the A2 colonists plan to overcome these dead-ends in those cases where they can. The last one, A3, is only interested in star travel when there is a threat to its survival on its home planet. They have to move to avoid extinction or at least destruction of their civilization. Having a few remnants of the species continuing to live through the catastrophe and go on for some time is not the plan they have for their civilization. They want a new world to clone the old one.
There has to be a very long time needed by the alien civilization for the design and construction of their space ship, and during that time, observations of the proposed destination solar system can continue. If there are multiple candidates, the first task is to select one, based on the tradeoff between distance and suitability. Observations can be done in any part of the electromagnetic spectrum, but the visible and near-visible, meaning infrared and ultraviolet, are probably the only ones where observations of an exo-planet make much sense. So they would do the best possible job of figuring out all they can about the potential destination planet, meaning a large dish in space, with long observation times, and everything possible extracted from the spectrum and time variations they observe.
As an example, assume they build parabolic reflectors somewhere in orbit around their home star, and just sit and observe until there is nothing else that can be extracted from the data. This might mean an aperture of hundreds of meters, or even kilometers, with observation times running for decades or even centuries. Perhaps there are several of these giant telescopes. Computational power is of no consequence; they have what they need.
They would understand the star of the destination planet well, and would be able to understand its stability, and if it was prone to any nasty events which might make the exo-planet destination uninhabitable. The number, size and location of other planets in that distant solar system would be well known, and some information on the details of the planets would be available, such as what types of atmospheres they had, if any, and the surface temperature of those not covered by an opaque atmosphere. It would be easy to estimate the orbital stability of the solar system’s planets, but asteroids would likely not be seen, so the question of how many of them crossed the destination planet’s orbit would not be available remotely. Conceivably, the larger moons in the solar system would be detected, and some information about them gleaned.
On the destination planet itself, the atmosphere in general would be detected, although layering might not be visible. They would know the composition of it, and if there were extreme winds. Cloud cover and its variability would be known. It would be possible to know if there were oceans on the surface, or if the planet was dry. This type of research would be a combination of the development of a specific model of the planet and its thermal and chemical characteristics, and matching or conflicting observations. The determination of what type of atmosphere is reasonable for a planet of a given size, albedo, and distance from its sun, along with the sun’s characteristics, is not complicated, and would be done by an alien civilization quite handily. Thus, before any starship was built, they would know the principal characteristics of the planet.
Since oxygen is produced by photosynthesis, they would know if this stage of evolution had been reached. Earlier stages, where there are chemotrophs somewhere in a body of water, probably do not give off any detectable signature, so if the planet were either barren, or in an early stage of evolution, prior to chlorophyll arriving on the stage, they could not tell this. However, if there was oxygen, they would also look at the signature of the land masses, and could tell if they were covered with green vegetation. Of course, we do not know if there is any other color of vegetation, in other words, a chlorophyll substitute that absorbed photons in different parts of the spectrum than chlorophyll does, but this is simply a matter of chemistry, and the alien civilization would be able to perform the same measurements, no matter what chemical the exo-planet had chosen for photosynthesis. The question of whether photosynthesis is likely or not is fairly well determined by the lack of other intense energy sources for life to take advantage of. It is either chemical energy, likely in solution, or photosynthesis.
The orbital parameters of the planet would be easily determined. They would understand the axial tilt, the eccentricity, and possibly the cyclic variation of them to some degree. This is a predictor of seasons, and if the temperature is correct, there should be a variation in averaged albedo over the year. Watching this periodic variation for many years should give them an idea of whether or not the planet was in an ice age. Since planetary climate can be unstable, the same planet can be in an ice age or not in one, as this status fluctuates back and forth.
It might even be possible for them to detect a magnetosphere around the planet, and determine if it has a magnetic field and how strong it was. The details of how this might be detected is left to the reader.
So, long before an alien civilization sets out on an epic voyage to another star, with the attendant costs and risks, they would have a great deal of information about their proposed destination. The information would be confirmed in most cases, and the arrivals would be assured of no surprises. Astronomical observations cannot determine certain things about exo-planets, and that must be left to probes. Since it is quite possible to radio back messages from star to star, over some reasonable separations, the logical next step for any planet, A1, A2, or A3, would be to send a one-way probe.
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