One thing that makes these particular planets a bit easier to find than usual is that they are young; HR 8799 and its children are only about 60 million years old. That means the planets are still glowing from the leftover heat of their formation, and that adds to their brightness. Eventually (in millions of years), as they cool, they will glow only by reflected light from the star, and be far harder to see. Fomalhaut b, in the Hubble image, is much older (200 million years), and glows only by reflected light from Fomalhaut. If it were much smaller or dimmer (or closer to the blinding light of the star), we wouldn’t have been able to see it at all.
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