S&P 500 $^GSPC is changing its composition and it shows what the market now considers the future and what the past.

Starting Monday, September 21, the index will replace three new companies: Bloom Energy $BE, Illumina $ILMN and Everpure $P. Out go Molson Coors $TAP, Builders FirstSource $BLDR and The Trade Desk $TTD, which will move to the smaller SmallCap index.

The most interesting is Bloom Energy, a fuel cell maker that waited several quarters for inclusion and has finally made it. The company benefits from AI data centers' hunger for electricity, the stock has nearly tripled in value this year and market capitalization exceeded $74 billion. After the inclusion news, it added over 7% in after-hours trading. It was by far the largest company waiting to enter the index.

Illumina, maker of DNA sequencing technology, and Everpure, operator of cloud and data storage, each have around $33 billion. The outgoing trio, on the other hand, each has around $7 billion.

This size mismatch will force the index manager to sell a piece of all remaining companies to free up room for the newcomers' weight. Also interesting is what didn't get in, even though it was expected. Analysts had also tipped Cheniere Energy $LNG and Twilio $TWLO, both lost this bet.

I take it as a quick barometer of market sentiment. The question is whether AI energy like Bloom Energy will last longer in the index than the companies it just replaced did.