The vex tech is the short answer to your question. It was discussed from the lore in D1 that whatever the vex did to mercury, caused the sun to appear dimmer from the planet’s surface. This likely indicates a diffusal effect similar to how the sun and moon can appear larger when on the horizon. The atmosphere distorts and expands the image of the sun/moon as we see it and as a result, appears both larger and less bright because of the image being spread out over a larger space than normal. In the case of mercury, it’s been proposed that the planet itself was diffused and now exists in more dimensions than it did previously. So, as a result, the effects of the sun are now spread out over all of those dimensions. Of course, bungie overlooked this when they made up the image of Mercury’s past painting it as an oasis of life. Since for the traveler to make that possible, it would have to have the planet no longer be tidally locked(give it a rotation, to prevent one side being an instant bake oven at 450F and the other a deep freeze at -170F) while also changing the magnetosphere of the planet to ward off the intense radiation of the sun itself. As to support a stable atmosphere. And probably some other stuff I’m overlooking on the subject of terraforming. In short: bungie, y u do dis?