Astronomy: Five planets will shine in the November skies
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The astronomer Rafael Bachiller reveals to us in this series the most spectacular phenomena of the Cosmos. Topics of exciting research, astronomical adventures and scientific news about the Universe analyzed in depth.
A planet has been detected roaming the Milky Way that is not linked to any star and that has a mass less than that of Earth.
It is the smallest of the wandering planets known to date.
Wandering through the interstellar medium
Computer simulations show that, in the formation of a planetary system, some of the planets can be thrown into interstellar space, losing their bond to the star with which they formed.
It is thus predicted that there must be many planets that roam interstellar space, without being linked to any star.
In fact, a couple of dozen objects are known to have properties that fit well with theoretical predictions and are good candidates for orphan planets.
But the study of such wandering planets (and worth the redundancy) is very difficult because no planet has its own light and, not being close to any star, they also have no nearby light to reflect.
They are small, dark objects that, for now, can only be detected using a very special technique called 'gravitational microlensing'.
Microlenses
In this method, you need to keep an eye on many stars in a celestial field and wait for some of them to experience a little brightening.
This brightening, if it cannot be attributed to other causes, is thought to be due to the passage in front of the star, exactly in the same line of sight, of a planetary object.
The study of brightening allows to determine the mass of the planet that passes in front and some other characteristics.
As we can see, microlenses should be very rare phenomena, since the perfect alignment of a star with the planet and the Earth is required.
It is thought that, observing just one star, we would have to wait a million years for an orphan planet to pass in front of us and we can see the microamplification of starlight.
But with wide-field telescopes, many stars can be observed simultaneously in a single image, and as a result, the probability of hunting one of these microlenses is greatly increased.
42 minutes
The OGLE (Optical Gravitational Lensing Experiment) project has been using, for almost 30 years, a small telescope equipped with a 1.3 meter diameter telescope in Las Campanas (Chile) to monitor millions of stars around the center of the Milky Way every night that the sky is clear (which is almost always there).
The project, led by the University of Warsaw, has made it possible to detect many variations in brightness in stars that can be due to multiple causes: intrinsic variability, presence of companion stars or planetary systems, etc.
Of course, the project also looks for microlensing phenomena.
Microlens brightening caused by OGLE-2016-BLG1928ApJLett / P.
Mróz et al.
The fastest microlens detected to date, known as OGLE-2016-BLG-1928, only lasted 42 minutes.
It was observed on June 18, 2016, in addition to OGLE, with the Korean Microlens Network (KMTNet) which has telescopes deployed in Chile, Australia and South Africa.
To explain such a rapid phenomenon, the team of astronomers coordinated by Przemek Mróz, a postdoctoral researcher at Caltech and the University of Warsaw, concludes that the object causing the microlensing must be a small orphan planet with a size between Mars and the earth.
And its mass seems to be closer to that of Mars than to that of our blue planet.
However, its distance cannot be calculated from the available data.
In the light curve of the microlensing, there is no indication of a star around which this small planet could orbit.
If the planet had been orbiting a star, its distance from it would have been greater than 8 times the Earth-Sun distance.
In our solar system, this orbit would be located between those of Jupiter and Saturn.
Lighter than Earth
The orphan planets known to date were much larger, even larger and more massive than Jupiter.
So the new detected object would be the smallest of the known orphan planets.
However, small planets (with masses similar to that of the Earth, or even up to a third of this value) seem more likely to be ejected from their planetary systems due to gravitational interactions with the gaseous giants that are part of these systems. .
OGLE-2016-BLG-1928 seems like a good representative of this type of small orphan planets that can be very abundant in the Galaxy, but very difficult to observe.
NASA has designed the "Roman" telescope that could be launched around 2025 with the aim of conducting a comprehensive study of microlenses.
The new telescope is named after Nancy Roman, the astronomer who is considered the 'mother' of the Hubble Space Telescope for the pivotal role she played in its design.
It is estimated that with the Roman telescope, astronomers would be able to detect about 250 orphan planets, of which about 60 would be terrestrial, or even lighter.
The article by Mróz et al., Entitled "A terrestrial-mass rogue planet candidate detected in the shortest-timescale microlensing event", has been published in a recent issue of the North American magazine The Astrophysical Journal Letters.
The manuscript can be consulted here.
Rafael Bachiller
is director of the National Astronomical Observatory (National Geographic Institute) and academic of the Royal Academy of Doctors of Spain.
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