Exactly who or what a "Solarbot" was intended to be is unknown. An early proof card of the Generation 2 Gobots Firecracker, Gearhead, and Blowout toys features the blurb "FREE* Solarbot Transformer (see back for details)."
On the final cards, this same blurb area remains, but now reads "HIGH-SPEED RACING AXLES!"
At this time, no actual "back" of the proof card is known to exist. Hopefully someday we will learn more. Until then Solarbot remains a mystery...