Tantalizing evidence has been uncovered for a mysterious population of
"free-floating" planets, planets that may be alone in deep space, unbound to
any host star. The results include four new discoveries that are consistent
with planets of similar masses to Earth, published today in Monthly Notices
of the Royal Astronomical Society.
The study, led by Iain McDonald of the University of Manchester, UK, (now
based at the Open University, UK) used data obtained in 2016 during the K2
mission phase of NASA's Kepler Space Telescope. During this two-month
campaign, Kepler monitored a crowded field of millions of stars near the
center of our Galaxy every 30 minutes in order to find rare gravitational
microlensing events.
The study team found 27 short-duration candidate microlensing signals that
varied over timescales of between an hour and 10 days. Many of these had
been previously seen in data obtained simultaneously from the ground.
However, the four shortest events are new discoveries that are consistent
with planets of similar masses to Earth.
These new events do not show an accompanying longer signal that might be
expected from a host star, suggesting that these new events may be
free-floating planets. Such planets may perhaps have originally formed
around a host star before being ejected by the gravitational tug of other,
heavier planets in the system.
Predicted by Albert Einstein 85 years ago as a consequence of his General
Theory of Relativity, microlensing describes how the light from a background
star can be temporarily magnified by the presence of other stars in the
foreground. This produces a short burst in brightness that can last from
hours to a few days. Roughly one out of every million stars in our Galaxy is
visibly affected by microlensing at any given time, but only a few percent
of these are expected to be caused by planets.
Kepler was not designed to find planets using microlensing, nor to study the
extremely dense star fields of the inner Galaxy. This meant that new data
reduction techniques had to be developed to look for signals within the
Kepler dataset.
Iain notes: "These signals are extremely difficult to find. 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. From that cacophony, we try to extract tiny,
characteristic brightenings caused by planets, and we only have one chance
to see a signal before it's gone. It's about as easy as looking for the
single blink of a firefly in the middle of a motorway, using only a handheld
phone."
Co-author Eamonn Kerins of the University of Manchester also comments,
"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. Now it passes the baton on to other missions that
will be designed to find such signals, signals so elusive that Einstein
himself thought that they were unlikely ever to be observed. I am very
excited that the upcoming ESA Euclid mission could also join this effort as
an additional science activity to its main mission."
Confirming the existence and nature of free-floating planets will be a major
focus for upcoming missions such as the NASA Nancy Grace Roman Space
Telescope, and possibly the ESA Euclid mission, both of which will be
optimized to look for microlensing signals.
Reference:
Kepler K2 Campaign 9: I. Candidate short-duration events from the first
space-based survey for planetary microlensing, Monthly Notices of the Royal
Astronomical Society (2021), DOI:
10.1093/mnras/stab1377
Tags:
Space & Astrophysics