• Earth's second 'Trojan asteroid' has been discovered by a team of astronomers
  • Trojans are space rocks that share the same orbit around the sun as a planet
  • Had only been seen at Jupiter, Neptune and Mars prior to first Earth one in 2010
  • Second, named 2020 XL5, will be trapped in same orbit as Earth for 4,000 years
  • It is 1,800ft-wide and is a great candidate for future fly-by missions, experts say