The red dwarf star itself emits only about 0.4% of our sun’s radiance, so the planet receives about 2% of the intensity that Earth receives from its sun. This is because Barnard’s star is in the class of M dwarf stars, cooler and less massive than our sun. It’s also an old star that predates our own solar system.
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And it will still be shining long after the Sun is burned out.
Red dwarfs stay in their main sequence for trillions of years.
I am of the opinion that it would be easier to heat a cold planet via solar reflectors than to cool a hot planet. Either way, habitation if possible would require some terraforming.