While we have rivers on Earth, the term "river" is usually used to describe flowing water. On other celestial bodies in our solar system, we don't have rivers in the traditional sense.

However, there are some features that resemble rivers, but instead of flowing water, they consist of other materials:

* Titan's methane rivers: Titan, a moon of Saturn, has rivers and lakes filled with liquid methane and ethane. Some of these "rivers" are quite long, with the largest being Vid Flumina, which is estimated to be around 400 kilometers (250 miles) long.

* Lava flows on Mars: While not technically rivers, volcanic eruptions on Mars can create long, winding channels of solidified lava, similar in appearance to rivers. Some of these are quite extensive, though their length is difficult to measure precisely.

So, while we can't say definitively which is the "longest river" in the solar system, Vid Flumina on Titan is a strong contender for the longest known liquid flow.