In 2009, the first group of astronomers found a fourth planet, named 581e. They measured the planet's mass to be 1.94 times the mass of the Earth, and measured the planet's orbit to be 3.149 days long. The astronomers also studied planet 581d again, and made more accurate measurements for that planet. They found that the planet has a mass that is 7.09 times the Earth's mass, and that its orbit lasts for 66.8 days.