‘Lonely’ Rogue Planet Discovered While Wandering The Milky Way

Researchers believe our galaxy is teeming with cosmic orphans, wandering planets without a parent star. Although common, these rogue planets are difficult to spot, especially when they are the size of Earth.

Despite this difficulty; an international team of astronomers including Przemek Mróz, postdoctoral researcher at the California Institute of Technology (Caltech) and Radosław Poleski from the University of Warsaw Astronomical Observatory, have spotted what they believe to be a floating planet with a size and mass somewhere in the range of Mars and Earth, roaming the Milky Way.

The discovery represents a big step forward in the field of exoplanet investigation, as it is the first Earth-sized “rogue planet” ever to be observed.

Artist’s impression of a gravitational microlens event by a free-floating planet. (Jan Skowron / Astronomical Observatory, University of Warsaw)

“We have found a planet that seems extremely lonely and small, very far in the Universe,” Poleski says. ZME Science. “If you can imagine, Earth is in a sandbox surrounded by lots of other planets and sunlight. This planet is not. It is really alone.

The rogue planet the team found – OGLE-2016-BLG-1928 – is believed to be the smallest floating planet ever to be discovered. It was found in data collected by Optical Gravitational Lensing Experiment (OGLE), a Polish astronomical project based at the University of Warsaw. Previously discovered thieves – like the very first ever-recorded floating planet also found by OGLE in 2016 – are closer in size to Jupiter.

The gravity of a floating planet can deflect and focus light from a distant star as it passes nearby in front of her. Due to the distorted picture, the star temporarily looks much brighter. (v)

“We have discovered the smallest floating candidate planet to date. The planet is probably smaller than Earth, which is consistent with the predictions of the theories of the formation of planets ”, Mróz – lead author of the team’s study published in Letters from astrophysical journals– explains to ZME Science. “The floating planets are too faint to be observed directly – we can detect them using gravitational microlenses via their light bending gravity.

The seriousness of the situation

The team spotted this wandering planet using the gravitational microlens technique, often used to spot exoplanets – planets outside of our solar system. Exoplanets often cannot be observed directly, and when they can, it is the result of interacting with radiation from their parent star – for example, the gradation effect of exoplanets as they intersect in front of their star and block some of the light it emits. Obviously, since rogue planets don’t have a parent star, they don’t have these interactions, which makes micro-lens events the only way to spot them.

“The microlens occurs when a lens object – a free-floating planet or star – passes between an Earth-based observer and a distant source star, its gravity can deflect and focus the light from the source,” Mróz explains to ZME Science. “ The observer will measure a brief brightening of the source star, which we call a gravitational microlens event. “

When the gravity of a floating planet deflects and focuses light from a distant star, we can observe temporary changes in the star’s brightness.(temporary changes in star brightness.
Credit: Jan Skowron / Astronomical Observatory, University of Warsaw. )

Mróz goes on to explain that the duration of microlens events depends on the mass of the object acting as a gravitational lens. “The less massive the lens, the shorter the microlens event. Most of the observed events, which usually last for several days, are caused by stars, ”Mróz explains. “Microlens events attributed to floating planets typically last just a few hours, making them difficult to spot. We have to observe the same part of the sky very frequently to spot brief highlights caused by floating planets. “

Changes in the star’s brightness observed during the microlens gravitational event by a free-floating planet.(Credit: Jan Skowron / Astronomical Observatory, University of Warsaw/ Robert Lea)

By measuring the duration of a microlens event and the shape of its light curve, astronomers can estimate the mass of the lens object. This is how the team was able to determine that this floating planet was approximately the size of Earth. “As a result, we can discover very dark objects, like black holes or floating planets,” Poleski explains. “We found it to be an event that was 41 minutes long. And it is the shortest event ever discovered.

Poleski explains that the absence of any other lens body in the system indicated to the team that this was a very good candidate for a floating planet. He adds: “We know it’s a planet because of its very short timescale and we think it floats freely because we don’t see any stars next to it.”

Become Snape. How the floating planets come to roam the universe on their own

Astronomers believe that floating planets actually formed into protoplanetary disks around stars in the same way “ordinary” planets did. At some point, they are ejected from their parent planetary systems, possibly after gravitational interactions with other bodies, for example with other planets in the system.

“Some low-mass planets are expected to be ejected from their parent planetary systems during the early stages of the formation of the planetary system,” Mróz explains. “According to the theories of the formation of the planets, most of the ejected planets should be smaller than the Earth. Theories of planetary formation predict that the typical masses of ejected planets should be between 0.3 and 1.0 Earth mass. Thus, the properties of this event correspond to theoretical expectations.

These free-floating rogue planets are believed to be quite common, but researchers cannot be sure because they are so difficult to spot. “Our current studies indicate that the frequency of low-mass planets – in the mass range from Earth to super-Earth – free-floating or large-orbiting is similar to that of stars – there are about two to five of these. objects for every star in the Milky Way, “Mróz says.” These numbers are very uncertain as they are based on a few observations of short-term microlens events. However, if floating / wide-orbiting planets were less common than stars, we would have observed far fewer short-term events than we did.

The researcher adds that although these objects are relatively common, the chances of observing microlens events caused by them are still extremely low. “Three objects – the source, the objective and the observer – must be almost perfectly aligned,” explains Mróz. “If we were to observe only one source star, we would have to wait almost a million years for the source to be microlensed.”

In fact, one of the extraordinary elements of the team’s study is that such a short-lived lens event was not considered observable given the sensitivity of the current generation of telescopes.

“The surprise, in general, was that with current technology we could define an event of such a short duration,” Poleski explains. “It’s especially surprising if you break the previous record by a factor of a few. “

The Nancy Grace Roman Telescope and Future Rogue Reconnaissance

For Mróz, there are still questions he would like to see answered regarding OGLE-2016-BLG-1928. Mainly, confirming that it is indeed a free-floating planet.

“We are not entirely sure whether our planet floats freely or not. Our observations rule out the presence of stellar companions within 10 astronomical units – 930 million miles – of the planet, but the planet could have a more distant companion, ”Mróz explains. “Imagine we are observing microlens events through a solar system lookalike. If Jupiter or Saturn caused a microlens event, we would see a signature from the Sun in the light curve of the microlens event. However, the microlens events of Uranus or Neptune would likely resemble those of the floating planets, as they are very far from the Sun. “

Fortunately, Mróz says it should be possible to distinguish between floating planets and wide orbiting planets. “The lens moves relative to the source star in the sky and – a few years after the microlens event – the lens and the source should separate in the sky,” says the researcher. “If the objective has a stellar companion, we will see excess light at its position. If it’s a floating planet, we won’t.

While this method may seem simple, Mróz says we can’t apply it now because existing telescopes aren’t powerful enough. This includes the instrument that made the long-term observations that gave rise to the OGLE sky study – the data from which the team found the OGLE-2016-BLG-1928 micro-lens event.

“[The discovery of OGLE-2016-BLG-1928] was part of the larger research of microlens events in general, which we are doing in stages, ”said Poleski ZME. “In one step, I started looking at the wide orbit planets – planets similar to Uranus or Neptune and in similar orbits. And while researching them, I sifted through a list of candidate microlens events in general and found this one.

Soon, NASA’s Nancy Grace Roman Telescope will take over the search for microlens events, but in the meantime, there is still data from Google and other projects to consider. “We now have more data and other surveys are also collecting data. So we hope to analyze them, ”says Poleski. “The longer term future is the launch of the Nancy Grace Roman Space Telescope. It will be a telescope similar to the Hubble Telescope, only with new infrared and infrared cameras and that camera field of view larger than the Hubble Space Telescope.

“One of the main projects of the Raman Telescope will be to observe the galactic bulge for microlens planets, including floating planets.”

Mroz, P., Poleski, R., Gould, A. et al., ‘Candidate landmass rogue planet detected in shortest microlens event,’ Letters from astrophysical journals, [2020] DOI: 10.3847 / 2041–8213 / abbfad


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Arline J. Mercier

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