Assuming everything with that color is the same substance, I'm unable to think of any combination of forces that would create that shape. Much less can I guess what the substance is. Any ideas?
Fire aboard a ship, two or three ships to be more specific.
From your photo (artwork) it's difficult to tell, and the same art backdrop is reused for the view on screen of Xahea and the sensor information:
It costs less to reuse the same artwork.
My only partial guess is that it has something to do with dilithium mining. If that were a huge plume of waste from the mining process, and if that waste were electrostatically charged, then I suppose the planets magnetic field might draw the material to where it circles the equator like that.
That we can rule out, there is no "plume" from the ground rising up into space from the equator. See these two similar closeups, one with Sarek's ship:
Xahea and Sarek's ship
Note that the ring doesn't originate from the ground nor circle the equator.
The 'string' is a ship on fire leaving or crashing on the surface, it's travel speed (indicated by the smoke diameter) is constant, it's probably a crash. Notice that it appears to twist (like a spore drive?) and be traveling at a much higher speed the the other smoke trails.
Note the third plume with the large ball on the end (and the second white hot spot), that indicates that the ship is coming to a halt.
Here's a closeup:
Click to zoom in
There might be a fourth plume (the smallest diameter) from the lower bright white dot passing under the upper one, at a constant altitude from the planet, leading to the largest blob of the ring; it could be part of a ship blown away.
Also notice the bright blue dot left of the lower white dot, it could be an engine; it appears to be the source of illumination of the circular spot on the planet. The planet's sun would light a much larger portion of the surface than such a small sector. The smoke ranges in diameter from a few to perhaps ten times larger than the white dots.
One comment put forth a theory (without links or math) proposing that the smoke is 10's of thousands of miles in diameter, so it also must be 10 or 20 thousand miles out in space (based on the comment versus what the image shows). That's 1/10 to 1/20th the distance to the moon. Xahea is listed on Memory Alpha as a Class M planet, with a diameter between 10,000 and 15,000 kilometers (6213.712 to 9320.568 miles). The smoke isn't larger than the whole planet, we can see most of one side of the planet.
Here's the quotation from the webpage where those images were obtained ('Feed Format', rename by adding ".html" suffix):
"Is Sarek coming back?
It is not uncommon in Star Trek to have the heroes facing off against a powerful enemy alone, and we know that Control had infiltrated Starfleet subspace communications to prevent any calls for reinforcements. Michael’s adoptive father Sarek showed up halfway through that episode because he could track her using his special katric link. After he and Amanda said their goodbyes, they left before the battle with the Section 31 fleet started. Since Sarek is a high-level ambassador for the Federation and now knows exactly where this battle is going to take place, is he going to take his shuttle to Earth or any other Starfleet hubs to gather some reinforcements? If he doesn’t, he very well may be the worst dad ever.
Sarek’s shuttle left the Disco before the Section 31 fleet arrived, but will he come back with help?".