A large soletta can be used to increase the total amount of solar radiation reaching the planet, but only slightly and under special conditions. The soletta needs to be larger than the planet and with an orbital inclination near identical to the planet. Even then, the soletta will only be functional during conjunction, when the soletta and planet are aligned in their orbit around the sun. The closer the soletta is to the planet, the longer between conjunctions. The closer the soletta is to the sun, the larger it must be. Considering it's vast size, the significant resources required to produce it, the energy involved in construction, launch and ongoing maintenance, and compared to the negligible increase in solar radiation, a soletta is simply inefficient and impractical. In addition, no currently known translucent material possesses sufficient strength to support a large soletta against solar wind, coronal mass ejections and gravitational collapse.
A small soletta does not gather additional sunlight. It simply redirects sunlight that would otherwise fall on the planet. What it does do is produce a localised temperature increase, intensely heating a small area by proportionally cooling a larger one. Given the long duration between conjunctions, the heated area has more than enough time to disperse the additional heat into the surrounding cooler area. This leaves only one possible advantage, the decomposition of minerals. All non-elemental minerals, given enough heat, will decompose. Many minerals decompose by releasing gas and forming more stable solids. These gases can contribute to the greenhouse effect, or to global dimming, or have no direct effect on global temperature at all.
The idea of warming a cold planet by intense localised heating can be misleading.
Yes, intensely heating a section of a planet can release greenhouse gases.
Yes, greenhouse gases will warm a planet.
Yes, the greenhouse induced temperature increase will release more greenhouse gases which will, in turn, raise temperatures higher, producing a positive feedback loop.
No, it's not the end result. The end-result is to produce a terraformed planet, in all respects, not just temperature.
No, the system is not stable and will likely return to its original state.
On any planet, artificial heating to release greenhouse gases is unnecessary. This becomes apparent when other steps are completed beforehand.
To use Mars as an example: The planetary mass of Mars must be drastically increased, in practical terms there just isn't enough spare high-density mass in the solar system to do this and so terraforming Mars remains a purely fictional topic. A complete Nitrogen atmosphere must be imported, from the outer system at considerable expense. Then a complete hydrosphere must be imported from the far outer system, also at considerable expense. Water is a greenhouse gas. As the planet warms CO2 is released from carbonate mineral decomposition, producing a positive feedback loop.
A completely terraformed planet will naturally warm to a desirable level on its own, as it was designed to do, and as it must do to remain stable.
So there's no need for an expensive, resource consuming soletta.
The presence of a soletta, is in fact, a sign of terraforming failure.