US scientists announced on Tuesday that the new discoveries were both about 15 to 20 times the mass of our own planet.
  
Geoffrey Marcy, of the University of California at Berkeley, told journalists: "Both planets orbit very nearby stars in our solar neighbourhood, and indeed they are only about 35 light-years away."

That's a mere 199 trillion miles. One of the two planets orbits the star 55 Cancri, already known for its three gaseous giants. The new planet's mass is 18 times that of Earth - slightly more than Neptune's.
  
The other new planet orbits star Gliese 436, and has a mass 20 times Earth's. 
  
Hard to spot

"These two new planets have sizes - diameters - that are in fact comparable to that of the Earth," March said at the press conference organised by NASA.

"They are only two or three times the diameter of our Earth." 
  

"I think it is fair we are poised unexpectedly for the next step in planet discovery, namely finding truly Earth-mass planets"

Geoffrey Marcy, 
University of California, Berkeley

"With these 15-Earth-mass planets that we are now discovering, I think it is fair we are poised unexpectedly for the next step in planet discovery, namely finding truly Earth-mass planets," he said.
  
"They are very difficult to detect, planets just the size of our Earth."
  
The difficulty is technical.

More discoveries

Astronomers do not actually see the planets, but can surmise their size and mass by measuring the gravitational wobbles they cause in the stars they orbit. Sharper telescopes and space probes will be needed to discover smaller planets.
  
Last week, European astronomers also announced the discovery of two planets beyond our solar system.

One, detected orbiting a star in the Autel constellation, has a mass 14 times that of Earth - making it the smallest planet ever found outside the solar system.
  
More than a hundred planets have so far been discovered beyond the nine planets around our Sun.