Air travel is thought to be responsible for 3% of the world's carbon emissions. Experts have called on the industry to reduce emissions, and the European Union has just said it will spend almost $3.5 billion to help airlines use more sustainable fuel. For now, that may be the best way for airlines to get a little greener, but there might be another option: hybrid planes. Right now, this is something that's still quite new, but more and more companies are exploring it. Hybrid planes work in a similar way to hybrid cars, which have been around for some time. But as electric cars are so popular, you might ask why companies are not moving straight to electric planes. They have been trying, but progress has been slow. Hybrid planes are a useful stepping stone — they get a lot of power from their batteries but have fuel as a backup. A company called Heart Aerospace has been working on hybrid planes, and says that they're lighter and cheaper than a fully electric plane would be. And they can still use fuel if they need to fly longer distances or when the batteries run out. Heart Aerospace plans to run a test flight of its hybrid plane by 2026. It says it should be able to travel 200 kilometers using only electric power but 400 kilometers using both batteries and fuel. However, that would only get a plane from New York to Washington DC, about the same distance as between Tokyo and Kyoto in Japan. These planes won't be able to transport large numbers of people, either. Heart Aerospace reckons it could take just 30 passengers on that 400-kilometer trip. Another US-based company called Electra is busy growing its hybrid plane business, and thinks its aircraft will be able to travel 2,000 kilometers, but carrying only nine people. The future of air travel is coming, but don't expect change to happen at supersonic speed!