Our Galaxy may be teeming with rogue planets, gravitationally unbound to any star. An international team of scientists, led by Polish astronomers, has announced the discovery of the smallest Earth-sized free-floating planet found to date.
From the
Astronomical Observatory of the University of Warsaw
October 29, 2020 -- Over four thousand extrasolar planets have
been discovered to date. Although many of the known exoplanets do not resemble
those in our solar system, they have one thing in common – they all orbit a
star. However, theories of planet formation and evolution predict the existence
of free-floating (rogue) planets, gravitationally unattached to any star.
Indeed, a few years ago Polish astronomers from the OGLE team from the
Astronomical Observatory of the University of Warsaw provided the first
evidence for the existence of such planets in the Milky Way. Writing in Astrophysical
Journal Letters, OGLE astronomers
announced the discovery of the smallest rogue planet found to date.
Exoplanets can be only rarely directly
observed. Usually, astronomers find planets using observations of the light
from the planet’s host star. For example, if a planet crosses in front of its
parent star’s disk, then the observed brightness of the star periodically drops
by a small amount causing so called transits. Astronomers can also measure the
motion of the star caused by the planet.
Free-floating planets emit virtually no
radiation and – by definition – they do not orbit any host star, so they cannot
be discovered using traditional methods of astrophysical detection.
Nevertheless, rogue planets can be spotted using an astronomical phenomenon
called gravitational microlensing. Microlensing results from Einstein's theory
of general relativity – a massive object (the lens) may bend the light of a
bright background object (the source). The lens’ gravity acts as a huge
magnifying glass which bends and magnifies the light of distant stars.
If a massive
object (a star or a planet) passes between an Earth-based observer and a
distant source star, its gravity may deflect and focus light from the source.
The observer will measure a short brightening of the source star –
explains dr Przemek Mróz, a postdoctoral scholar at the California Institute of
Technology and a lead author of the study. Chances of observing microlensing are extremely slim because three
objects – source, lens, and observer – must be nearly perfectly aligned. If we
observed only one source star, we would have to wait almost a million year to
see the source being microlensed – he adds.
This is why modern surveys hunting for
gravitational microlensing events are monitoring hundreds of millions of stars
in the Milky Way center, where the chances of microlensing are highest. The
OGLE survey – led by Warsaw University astronomers – carries out one of such
experiments. OGLE is one of the largest and longest sky surveys, it started
operations over 28 years ago. Currently, OGLE astronomers are using a 1.3-meter
Warsaw Telescope located at Las Campanas Observatory, Chile. Each clear night,
they point their telescope to the central regions of the Galaxy and observe
hundreds of millions of stars, searching for those which change their
brightness.
Gravitational microlensing does not
depend on the lens’ brightness, so it enables the study of faint or dark
objects such as planets. Duration of microlensing events depends on the mass of
the lensing object – the less massive the lens, the shorter the microlensing
event. Most of the observed events, which typically last several days, are
caused by stars. Microlensing events attributed to free-floating planets have
timescales of barely a few hours. By measuring the duration of a microlensing
event (and shape of its light curve) we can estimate the mass of the lensing
object.
The scientists announced the discovery
of the shortest-timescale microlensing event ever found, called
OGLE-2016-BLG-1928, which has the timescale of just 42 minutes. When we
first spotted this event, it was clear that it must have been caused by an
extremely tiny object –says dr Radosław Poleski from the Astronomical
Observatory of the University of Warsaw, a co-author of the study. Indeed,
models of the event indicate that the lens must have been less massive than Earth,
it was probably a Mars-mass object. Moreover, the lens is likely a rogue
planet. If the lens were orbiting a star, we would detect its presence
in the light curve of the event – adds dr Poleski. We can rule
out the planet having a star within about 8 astronomical units (the
astronomical unit is the distance between the Earth and the Sun).
OGLE astronomers provided the first
evidence for a large population of rogue planets in the Milky Way a few years
ago. However, the newly-detected planet is the smallest rogue world ever
found. Our discovery demonstrates that low-mass free-floating planets
can be detected and characterized using ground-based telescopes – says
Prof. Andrzej Udalski, the PI of the OGLE project.
Astronomers suspect that free-floating
planets actually formed in protoplanetary disks around stars (as “ordinary”
planets) and they have been ejected from their parent planetary systems after
gravitational interactions with other bodies, for example, with other planets
in the system. Theories of planet formation predict that the ejected planets
should be typically smaller than Earth. Thus studying free-floating planets
enables us to understand the turbulent past of young planetary systems, such as
our solar system.
The search for free-floating planets is
one of the science drivers of the Nancy Grace Roman Space Telescope, which is
currently being constructed by NASA. The observatory is scheduled to start
operations in the mid-2020s.
Because of the brevity of the event,
additional observations collected by the Korea Microlensing Telescope Network
(KMTNet) were needed to characterize the event. KMTNet operates a network of
three telescopes – in Chile, Australia, and South Africa.
The discovery would be impossible
without long-term observations conducted by the OGLE sky survey. The project is
among the largest and longest sky surveys, it started operations over 28 years
ago. One of the first goals of the OGLE survey was searching for and studying
dark matter using the gravitational microlensing technique. Current studies
cover a large range of topics – searching for exoplanets, studying the
structure and evolution of the Milky Way and neighboring galaxies, studies of
variable stars, quasars, transients, and solar system bodies.
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