Four candidates are running: José Bové, Monica Frassoni, Rebecca Harms and Ska Keller. They've already gone through two stages before the open primaries - first they have to be nominated by a national party of the European Greens, and then get the support of at least four member parties. This threshold is lower than the PES's (of 6 member parties), and coupled with the maximum limit of 8 supporting member parties, probably encouraged more candidates to come forward and be selected.
The primaries will select 2 leading candidates for the European Greens who will represent them in the election campaign and in debates with other Europarty leaders.
José Bosé is a French Green MEP who has been in the European Parliament since 2009 and is Vice-Chair of the Agriculture and Rural Development Committee. He got into the Green movement as a farmer in France and has campaigned against nuclear power and GMOs. He is against fracking. Bosé is anti-globalisation and campaigned for a No vote in the French referendum on the Lisbon Treaty. He has criminal convictions for destroying GMO crops and a McDonald's in France.
Monica Frassoni is the current co-president of the European Green party. In the last parliamentary term (2004-9) she was a co-leader of the Greens/EFA group in the European Parliament, but did not retain her seat in the election. She supports a "Green New Deal" and is a European Federalist. There has been some controversy over Frassoni running in the Italian elections on a list that competed with the Italian Greens (the Left Ecology Freedom list), which caused the Young Italian Greens to demand her resignation from the European Greens co-presidency.
Rebecca Harms is a German Green MEP who currently leads the European Green/EFA group in the European Parliament in debates (along with Daniel Cohn-Bendit). Harms was a landscape gardener before becoming a politician, and has also led the Green group in the Lower Saxony state parliament between 1998-2004. She got into the Green movement through the anti-nuclear campaign in Germany (over the infamous nuclear waste dump in Gorleben) and has spoken on energy transition and climate change in the EP. Harms supports further EU integration. As co-chair of the European Parliamentary group, she'll probably top the primary.
Ska Keller is a German Green MEP and the candidate nominated by the youth wing of the European Greens. She grew up behind the Iron Curtain and has worked on cross-border solidarity in her home on the Polish border. Keller is a member of the Committee on International Trade and a substitute on the Committee on Civil Liberties, Justice and Home Affairs. Her specialisms in the EP are migration and EU-Turkey relations, and she is for better protection of refugees.
It's great to see an open primary get off the ground. Probably inspired by the French Socialist presidential primary that engaged people from outside the party, the European Greens will hope to get people interested first in the primary, and then in supporting the member parties come election time. It will be interesting to see how many participate.
If you're interested in voting, you can vote here.