Wandering planet: what is it?

Of the 5,656 exoplanets identified and confirmed in the catalog of exoplanets of the Paris Observatory teams (accessed April 30, 2024, editor’s note), there are a few which do not make any revolution around a star. ..

Is the simple organization of our solar system, namely a star and several planets orbiting it, the norm in the Universe or is it an exception? These floating planets, also commonly described as wandering or wandering, were named a few years ago with a pictorial expression, the “floating Jupiters”, because of their mass several times greater than our gaseous neighbor. How many are there? How were they formed? And how are they detected?

What is a wandering planet?

Scientific publications generally refer to these wandering objects by two abbreviations in English:

  • FFPs for free-floating planetsliterally translatable as free-floating planet.
  • IPMO for isolated planetary-massor planetary mass object.

Astronomers have known about them since the mid-1990s. These wandering worlds do not seem gravitationally linked to a star.

In 2013, a Hawaiian telescope spotted just 80 light years from Earth (one light year is 9.460 billion kilometers), a planet in its infancy, PSO J318.5-22, since it was formed just 12 million years ago.

It has all the attributes of a planet that orbits a star: mass, color and energy emitted, but is not linked to any visible star.

Brown dwarf or “floating Jupiter”: how to define the wandering planet by mass?

For us, laymen, the first surprise: these wandering planets shake up our strict definitions of a planet and a star. Their mass could be that of a giant planet or an aborted star, a brown dwarf.

Some observations have confused a fraction of them with brown dwarfs, stars of lower mass and larger than a gas giant planet. “Born in the same way as classical stars, by gravitational contraction of a nebula, [la] mass [d’une naine brune] is, however, too weak to be able to sustain the thermonuclear fusion reaction of hydrogen. [Elle] will have started to fuse the hydrogen, but due to its too low mass, its pressure and its temperature do not allow this reaction to be maintained, hence the term sometimes usedaborted star.

Brown dwarfs are considered intermediate objects between stars and planets, objects that coexist in the stellar clusters of planets without parent stars. Their discovery in the 1990s probably marks the start of a craze among astronomers for wandering worlds in the Universe.

SIMP J01365663 + 0933473, classified as a brown dwarf in 2016, was reclassified as a wandering planet two years later. Its mass is equivalent to approximately 13 times the mass of Jupiter, the most massive of the gas planets in our solar system, which has become the standard planet for scientific publications to express the mass of these FFPs.

WISEA 1147, a candidate for wandering exoplanet status, has been reclassified as a brown dwarf, although below the official limit. “The official mass limit of a brown dwarf is thirteen times the mass of Jupiter. But this limit is controversial. In fact, what really makes the difference between a planet and a failed star is the consumption of deuterium within them. Below a certain mass, no nuclear reaction and the deuterium is intact. Above, the fusion of the deuterium begins. explains planetologist Philippe Delorme to Science and Future (in our n°792, February 2013).

Wandering Earth-sized planets are hiding

There are probably wandering Earth-sized planets, but they are difficult to ferret out.

The discovery in 2020 of an unattached planet with a mass roughly equivalent to that of the planet Mars is therefore a first. Polish astronomers from the OGLE project discovered it by observing a peak in brightness and a distortion of space around this tiny wandering object during a gravitational microlensing phenomenon. The phenomenon was so brief, 41.5 minutes, that researchers deduced that the planet had a mass similar to that of Mars or Earth.

How many wandering planets could there be?

The existence of free-floating planets has been predicted by the theory of planet formation, but how many are there?

A Japanese astrophysicist estimated in 2011 that there could be several billion. In 2017, these predictions were revised downwards. There would rather be one wandering planet for every four stars and a total approaching a few hundred million (see Science and Future monthly, n°847, September 2017). This new count seems more in line with theoretical models of planetary formation.

How did these planets become wandering?

The question of the formation of these objects remains open. Several scenarios were considered:

  • Of the ” kicks ” gravitational. From simulations of the evolution of planetary systems, it was imagined that these unattached planets had been ejected from their planetary system then in full formation. They were born there, like the others, within a proto-planetary disk and then would have been ejected thanks to gravitational interactions or even by a neighboring planet. The planet jostled as a result would have lost its gravitational link with its star. In 2013, astronomer Sean Raymond explained to Science and Future : “We believe that this type of instability is very common and occurs in 50 to 90% of systems with giant planets“.
  • Born in a nursery of stars. The process imagined in this scenario would be similar to the birth of stars in vast nebulae made up of gas and dust. Lumps of matter appear under the effect of gravitational collapse leading to chain reactions leading to the formation of stars. Some objects formed within this cloud do not have enough mass to take this path. Their destiny would then be that of an IPMO. In 2023, researchers pointed the James Webb telescope at the Orion Nebula and were surprised to see stray Jupiter-mass objects coming in pairs. Could two planets be ejected as a pair?

Can there be an atmosphere on a wandering planet?

The proximity – relative, we are talking about several hundred light years of distance “only” – makes some of these wandering planets good candidates for the observation of their possible atmosphere by large terrestrial telescopes. CFBDSIR 2149-0403 discovered in 2012, is part of this category. Methane, water vapor and hydrogen were detected there.

The absence of a host star and its light was an asset for the observation of the atmosphere of CFBDSIR 2149-0403 and one can imagine that the wandering planets are ideal candidates for observation and analysis of planetary atmospheres.

The other observation criterion is the high temperature of these IPMOs which betrays their extreme youth. CFBDSIR 2149-0403, for example, is only 100 million years old, its temperature is around 430°C.

This is the criterion that astronomer Núria Miret-Roig, researcher at the University of Bordeaux, used in 2022 to study these floating objects. Among the 80,000 images collected, she focused on young, newly formed and still hot planets that shine in the first millions of years of their life, enough to bring together a harvest of 70 to 170 planetary mass objects! As for life, there is little chance of detecting any in IPMOs which are gas giants. “Some have satellites which could be more welcoming. But we know that life takes time to develop and here we are dealing with young objects. So there is not much chance of detecting anything“, affirmed the astronomer Hervé Bouy, co-signatory of the study by Núria Miret-Roig.

What techniques to detect wandering planets?

  • Direct observation is possible. The youth of certain FFPE goes hand in hand with a high temperature, which will be visible to infrared telescopes. The age is determined by the luminosity, and from this information the mass of the object is deduced (see Science and Future n°792, February 2013).
  • The other most common technique is that of gravitational microlensing. It consists of identifying, during direct observations, the effects of distortion of the light emitted by a star, caused by the passage of an object in front of this star. The gravitational field of these objects bends the path of the light of the star in the background. A bit like an object that we observe through a magnifying glass. The process assumes that we observe regions of the Universe rich in stars.

NASA is banking on the Nancy Grace Roman Space Telescope to detect these wandering planets. The instrument will be stationed a million kilometers from our planet, oriented in the opposite direction of our Sun, with the mission of finding planets the size of Mars. Enough to change us from the “floating Jupiters”.

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