This is "A Young Princess (Dorothea of Denmark?)" by Jan Gossaert. She seems to be playing with a golden object made of concentric rings, an armillary sphere showing the movement of heavenly bodies. Its interlocking circles are echoed in the decoration of her clothes.
The girl is probably about nine or ten years old and royalty, judging by the richness of her dress. The shape of the sleeves and the curve of the neckline suggest a date around 1530. The young girls in the Low Countries who might have appeared in such splendid clothes at this time were Dorothea and Christina, daughters of Christian II, exiled King of Denmark.
Dorothea indicates a point approximately 55° north of the equator on the sphere’s outer ring. This may be a clue to her identity - she's possibly directing our attention to her father’s lost kingdom. The contrast between the flat green background and the three-dimensional modelling of her figure makes her seem to project outwards towards us.
Gossaert painted Dorothea with her siblings in 1526, showing that this painting was likely made around 1530-2. It's an oil on wood panel, characteristic of the time period. Some believe it might have been commissioned by Margaret of Austria or one of the Habsburgs for whom Gossart often worked.
A Young Princess (Dorothea of Denmark?) hangs in The National Gallery on Trafalgar Square, London — free to enter. Point your phone at any artwork there and audioguide.london plays a free audio guide in six languages — no app download needed.