Artist’s impression of a free-floating “rogue planet.”
A. Stelter / Wikimedia Commons
Four planets thought to be of similar mass to Earth have been foundand none of them have a host star. 
These so-called rogue planets are free-floating, possibly after having been ejected from their star systems by the gravitational tug of heavier planets.
Published today in the Monthly Notices of the Royal Astronomical Society, the study used data from the K2 mission phase of NASAs Kepler Space Telescope, which despite having taken place in 2016 continues to herald astonishing discoveries. 
The researchers found 27 faint microlensing signals that lasted between an hour and 10 days, the four shortest of which are new discoveriesand the signals are consistent with planets of similar masses to Earth.
These planets also dont come with a telltale longer signal from a host star, suggesting that theyre rogue planets. 
They are about XXX light-years away in the constellations of XXXXX. 
What is gravitational microlensing?
The best way to discover far-off planets, microlensing describes how the light from a background star can be temporarily magnified by the presence of other stars in the foreground. The result is a short burst in brightness detectable by telescopes. Predicted by Albert Einstein’s General Theory of Relativity, microlensing occurs because the gravity of the foreground star will bend the light from the background star. Why? Mass bends space, so when light from the background star goes near the foreground objectin this case a rogue planet it takes a different path that if that planet wasn’t there.
 [These are] signals so elusive that Einstein himself thought that they were unlikely ever to be observed, said Eamonn Kerins of the University of Manchester and co-author of the paper. 
Only around one in a million stars in the Milky Way is visibly affected by microlensing while fewer still will be caused by planets. 
What was the Kepler Space Telescope?
“Kepler has achieved what it was never designed to do, in providing further tentative evidence for the existence of a population of Earth-mass, free-floating planets, said Kerins. 
Kepler was not designed to find planets using microlensing, nor to study the extremely dense star fields of the inner Galaxy. Launched in 2009, the Kepler Space Telescope was designed to calculate what fraction of stars in the Milky Way have Earth-size planets in their habitable zone. It observed almost 200,000 stars in the same tiny region of the night sky transiting their host stars, but technical problems that left it spinning meant that it had to be used to look fleetingly at different regions of the galaxy. The K2 Mission was born, and from April through July 2016 the telescope monitored a crowded field of millions of stars near the centre of the Milky Way every 30 minutes in search of gravitational microlensing events. 
Not that it was easy. Our observations pointed an elderly, ailing telescope with blurred vision at one the most densely crowded parts of the sky, where there are already thousands of bright stars that vary in brightness, and thousands of asteroids that skim across our field, said IAIN. From that cacophony, we try to extract tiny, characteristic brightenings caused by planets, and we only have one chance to see a signal before its gone.
Its about as easy as looking for the single blink of a firefly in the middle of a motorway, using only a handheld phone.
Artist’s impression of the Euclid spacecraft.
ESA/ATG medialab (spacecraft); NASA, ESA, CXC, C. Ma, H. Ebeling and E. Barrett (University of Hawaii/IfA), et al. and STScI (background)
The future of microlensing
Two upcoming space missions will look for exoplanets using the microlensing technique. One is the Nancy Grace Roman Space Telescopeor Roman, for shorta $3.2 billion space mission scheduled to launch in 2025 that will survey distant stars in the Milky Way. Its expected to find 100,000 exoplanets.  
The European Space Agencys EUCLID satellite, which will launch in mid-2022, will map the geometry of the Universe in an effort to better understand dark matter and dark energy, but also be capable of searching for exoplanets using the microlensing technique. 
Wishing you clear skies and wide eyes.