We will call the ancient astronauts the Gliesians. They lived in a six-planet solar system that was in orbit around a red dwarf star. Their home planet of Gliese was about 1.5 times larger than Earth. The planet they lived on had liquid water and was a rocky planet. It was a terrestrial planet much like Earth in the habitable zone of its star.
The Planet Gliese
The temperature on Gliese averaged from 0-40 Celsius under normal conditions. There was a Neptune and Jupiter sized planet in the outer part of that solar system and a Venus type planet closer to the star and two other smaller sized planets. However, all of Gliese’s oceans were beginning to get very cold as was the daily temperatures.
The red dwarf star at the solar system’s center had the mass of one half of that of our Sun. This solar system was located 21 light years from Earth in the constellation Cygnus.
The Red Dwarf Star of Gliese
The planet was beginning to suffer from bitter cold. The Gliesians began to make preparations for a migration. But where ? The planet next closest to its sun suffered from the greenhouse effect. The planets in the outer solar system were gas giants. They decided to send robot spacecraft to neighboring stars to look for a new home.
Gleisian Planetary Surveyor Robot
An M-class dwarf star such as the Gliesean’s star had a much lower mass than the Sun, causing the core region of the star to fuse hydrogen at a significantly lower rate. It had an effective temperature of 3250 Kelvin and it was falling due to age. It had a visual luminosity of 0.3% of that of the Sun.
However, this red dwarf radiated primarily in the near infrared, with peak emissions at a wavelength of roughly 900 nanometers, for the star's total luminosity. (For comparison, the peak emission of the Sun is roughly 530 nanometers, in the middle of the visible part of the spectrum). When radiation over the entire spectrum is taken into account (not just the part that humans are able to see), something known as the bolometric correction, this star had a bolometric luminosity 2.0% of the Sun's total luminosity. It was very dim by our standards.
This planet needed to be situated much closer to this star in order to receive a comparable amount of energy to that of a planet like Earth, which is what was needed there. But that energy was steadily decreasing. No one had any idea of how long the Planetary Surveyor Robot would take to find a suitable planet. Naturally, they would rely on astronomy and astrophysics to send them to the most likely areas in the galaxy, preferably some place that would not take long to reach. This would require a spaceship technology to have a warp drive technology at the very least. Then if the planet discovered already had intelligent life that would definitely complicate the picture. But they would have to migrate - there was no doubt about that. Time would be to short to do otherwise.
A city on Gliese during the daytime
Red dwarfs are the smallest, coolest, and most common type of star. Estimates of their abundance range from 70% to more than 90% of all stars in the galaxy, an often-quoted median figure being 73%. Red dwarfs are either late K or M spectral type. Given their low energy output, red dwarfs are never visible by the unaided eye from Earth; neither the closest red dwarf star to the Sun when viewed individually, Proxima Centauri (which is also the closest star to the Sun), nor the closest solitary red dwarf, Barnard's star, is anywhere near visual magnitude.
Planets that are close to red dwarfs to receive a sufficient amount of radiation for liquid water are likely to have long been tidally locked to their respective stars so that the planet rotates only once for every time it completes an orbit: this means that one face always points at the star (creating perpetual day) and one face always points away (creating perpetual night). Potential life would be limited to a ring-like region, known as the terminator, where the sun would always appear on the horizon. But Gleise was in a perfect spot in the habitable zone and did not have that problem.
Red dwarfs are far more variable and violent than than other stars. The light by them may be reduced down to 40% for months at a time and that can definitely effect the temperature on a planet. Often they are covered in star spots that can dim their emitted light. So this issue only made the problem worse for the Gleisians.
The Gliesian Red Dwarf Star with Star spots