Titan's Pixelbook

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The way your light shade bands the upper outline makes the sphere look really flat as only a line at the top appears illuminated like the brim of a coin.
You don't have to limit the use of yout ourline colour to the outline only, you can use it to shade, too.
If you want it to look more like a (stylized) sun, I'd suggest adding more pointy spikes instead of rather round petals.

Generally, I'd advice you not to work on a white background. Use a more neutral color instead (that doesn't clash with your sprite's colours). Moreover, the antialiasding pixels you out on the ourside will not work on different backgrounds as you can see in my image (top right), they will stick out - besides they don't make the lines look smoother and are not necessary on that scale.