- 4482 - FREE FLOATING PLANETS - how many are there? - Over 5,000 planets have been found orbiting other star systems. One of the satellites hunting for them is “TESS,” the “Transiting Exoplanet Survey Satellite”. Astronomers using TESS think they are made a surprising discovery; their first free-floating, or rogue, planet.
------------------------------- 4482
- FREE
FLOATING PLANETS - how
many are there?
- The rogue planet was discovered using
“gravitational microlensing” where the planet passed in front of a star,
distorting its light and revealing its presence.
-
- We are all familiar with the eight planets
in our solar system and perhaps becoming familiar with the concept of
exoplanets. But there is another category of planet, the “rogue planets”. These
mysterious objects travel through space without being gravitationally bound to
any star. Their origin has been cause for much debate but popular theory
suggests they were ejected from their host star system during formation, or
perhaps later due to gravitational interaction.
-
- Simulations have suggested that these
'free-floating planets' or “FFPs” should be abundant in the galaxy yet until
now, not many have been detected. The popular theory of ejection from star
systems may not be the full story though.
-
- It is now thought that different formation
mechanisms will be responsible for different FFP masses. Those FFPs that are
high mass may form in isolation from the collapse of gas while those at the low
mass end (comparable to Earth) are likely to have been subjected to
gravitational ejection from the system.
-
- Detecting such wandering objects among the
stars is rather more of a challenge than you might expect. Their limited
emission (or reflection) of electromagnetic radiation makes them pretty much
impossible to observe. Enter gravitational microlensing, a technique that
relies upon an FFP passing in front of a star, it's gravity then focusing light
from the distant star resulting in a brief brightness change as the planet
moves along its line of sight. To date, only three FFPs have been detected from
Earth using this technique.
-
- A team of astronomers have been using TESS
to search for such microlensing events. TESS was launched in April 2018 and
while in orbit, scans large chunks of sky to monitor the brightness of tens of
thousands of stars. The detection of light changes may reveal the passage of an
FFP as it drifts silently in front of the star. It's not an easy hunt though as
asteroids in our solar system, exoplanets bound to stars and even stellar
flares can all give false indications.
-
- One FFP candidate event associated with
the star “”TIC-107150013” is 3.2 parsec
away.
The event lasted 0.074 days
+/- 0,002 and revealed a light curve with features expected of a FFP. This
marks the first FFP discovered by TESS, an exciting step along the way to start
to unravel the mysteries surrounding these strange alien worlds.
-
- There's a population of planets that drifts
through space untethered to any stars. Some of these FFPs form as loners, never
having enjoyed the company of a star. But most are ejected from solar systems
somehow, and there are different ways that can happen.
-
- FFPs are mysterious because they're
extremely difficult to detect. But astronomers are getting better at it and are
getting better tools for the task. In 2021, astronomers made a determined
effort to detect them in Upper Scorpius and Ophiuchus and detected 70 of them,
possibly many more.
-
- In broad terms, there are two ways FFPs can
form. They can form like most planets do, in protoplanetary disks around young
stars. These planets form by accretion of dust and gas. Or they can form like
stars do by collapsing in a cloud of gas and dust unrelated to a star.
-
- For planets that form around stars and are
eventually kicked out, there are different ejection mechanisms. They can be
ejected by interactions with their stars in a binary star system, they can be
ejected by a stellar flyby, or they can be ejected by planet-planet scattering.
-
- The locations of 115 potential rogue planets
were recently discovered in 2021 by a team of astronomers in a region of the
sky occupied by Upper Scorpius and Ophiucus. The exact number of rogue planets
found by the team is between 70 and 170, depending on the age assumed for the
study region.
-
- Simulations have found that circumbinary
systems produce FFPs efficiently. In these simulations, each binary system
ejects an average of between two to seven planets with greater than 1 Earth
mass. For giant planets greater than 100 Earth masses, the number of ejected
planets drops to 0.6 planets ejected per system.
-
- The simulations also showed that most
planets are ejected from their circumbinary disks between 0.4 to 4 million
years after the beginning of the simulation. At this age, the circumbinary disk
hasn't been dissipated and blown away.
-
- The most important result might concern the
velocity dispersions of FFPs. As the
planets are ejected from the systems, they retain significant excess
velocities, between 8–16 km/s. This is much larger than observed velocity
dispersions of stars in local star-forming regions. So this means that the
velocity dispersions of FFPs can be used to tell ejected ones from ones that
formed as loners.
-
- The velocity dispersions provide another
window into the FFP population.
Simulations show that the velocity dispersion of FFPs ejected through
interactions with binary stars is about three times larger than the dispersion
from planets ejected by planet-planet scattering.
-
- They also found that the level of
turbulence in the disk affects planet ejection. The weaker the turbulence is,
the more planets are ejected. Turbulence also affects the mass of ejected
planets: weaker turbulence ejects less massive planets, where about 96% of
ejected planets are less than 100 Earth masses.
-
- We still don't have a strong idea of how
many FFPs there are. Some researchers think there could be trillions of them.
The upcoming Nancy Grace Roman space telescope will use gravitational lensing
to take a census of exoplanets, including a sample of FFPs with masses as small
as Mars'.
-
- In future work astronomers intend to
determine if there are chemical composition differences between FFPs. That
would constrain the types of stars they form around and where in their
protoplanetary disks they formed. That would require spectroscopic studies of
FFPs.
-
- Astronomers can begin to discern where
individual FFPs came from and better understand the population at large.
-
-
May 26, 2024 FREE
FLOATING PLANETS - how
many are there? 4482
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--------------------- --- Sunday, May 26, 2024
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