This is the translation of an article first published on the Italian blog Il Tredicesimo Cavaliere, created and maintained by Roberto Flaibani. This blog is one of the few Italian websites specialised in space research, astrophysics and SF, and it regularly features articles from Tau Zero Foundation, Centauri Dreams and other similar institutions. For more about Roberto and his amazing blog, see this. Here you have the English version of his post about nomad planets – mistakes are as usual all mine.
Nomad planets are not recent news, but they are not something that it can be easily forgotten either. Given that it’s almost a year since I haven’t read anything about them, I believe the readers of Il Tredicesimo Cavaliere would appreciate a recap of the situation. I will keep repeating it: the figures mentioned below in relation to the number of nomad planets existing in the Milky Way are shocking for their sheer magnitude and should be taken with caution. It’s also useful to recall that both gravitational microlensing and the existence itself of nomad planets are two relatively new fields of study. Perhaps the most appropriate attitude in these cases is Debra Fischer’s (Yale University): “They would be a kind of space couch grass” she told Nature, not without a touch of humour.
It seems nonetheless that these celestial bodies do exist, and that they roam in large numbers following the direction of rotation of the Milky Way. We now have some theoretical bases, which help us understand how and why these planetary bodies might have developed and eventually taken a hyperbolic orbit, leaving their own system. Researchers from the whole world have taken part in this investigation, but two groups in particular have given the most significant contribution to date: one directed by Abbot and Switzer from the University of Chicago, and the other led by Louis Strigari at Stanford University. One important thing has been clarified so far: in most cases, planets nomads originate in the protoplanetary disc along with all other bodies in star systems, and only later, in a subsequent, turbulent phase of adjustment they might be expelled as a result of a “gravitational kick”, which is a common occurrence in such occasions.
Hardware & numbers
The technique generally used for the detection of nomad planets is the so-called gravitational microlensing, which by its nature provides the best performance with targets of considerable size and far away from us, between 10,000 and 20,000 light years. Hence the estimate of 400 billion nomad planets in the Milky Way, with planetary masses not inferior to Jupiter. The ideal telescope for this type of research is going to be the long-awaited SKA, operative in 2020, which, according to Jonathan Nichols of University of Leicester, could discover at least 2,800 of them within an estimated range of 185 light-years. Moreover, the latest theoretical developments about planetary formation suggest that the ones with smaller mass might be expelled from their star systems more easily than the gas giants, and this fact would further increase the estimate of the number of planets nomads existing in the Milk Way.
Matthew Bernabé, an Italian astronomer who has taken part in the research, told Media-Inaf: “Our estimate of the number of nomad planets in our galaxy was calculated on the basis of ten newly discovered objects, through a technique called microlensing, in a small region of the galactic centre, i.e. near the centre of our galaxy. We have thus drawn the logical consequences regarding the global population of nomad planets, showing that for any main sequence star there might be up to 700 nomad planets with Earth’s mass and up to 100,000 nomads Pluto’s.” Too bad that planetary bodies smaller than Jupiter (those Earth-type, for instance) do not fall within the microlensing range, but rather into WFIRST (Wide-Field InfraRed Survey Telescope), an instrument much anticipated but that won’t be available before 2023.
Nomad planets – Formation
David Nesvorny of The Southwest Research Institute (SwRI) studied the possibility that even our solar system once saw the expulsion of a giant planet, which might have occurred when it was just 600 million year old. This is suggested by evidence found in the Kuiper Belt and on the Moon. At the beginning of the simulation the giant planets interact with the protoplanetary disk and end up in configurations in which pairs of neighboring planets block each other in an orbital resonance. This phenomenon occurs when two planets exert a regular, periodic gravitational influence on each other, as in the case of Jupiter’s moons Ganymede, Europa and Io, which are in resonance with each other in a ratio of 1: 2: 4. This means that Ganymede finishes one orbit in the time it takes Europa to complete two and Io four. Nesvorny suggests that these resonant systems become dynamically unstable once the protoplanetary disk gas is exhausted, while the planets assume instead eccentric orbits.
To get to its present state, the outer solar system must have gone through a violent phase of adjustment. The system has then stabilised by removing the excessive orbital energy present in the protoplanetary disk, whose fragments still survive to this day in the Kuiper Belt. The framework outlined here also suggests that Jupiter, starting from a position farther away from where it is now, has moved toward the centre of the solar system, spreading a quantity of planetesimals toward the Sun and into the opposite direction and wreaking havoc among the inner planets and the Earth’s Moon.
Nesvorny added to the model an additional giant planet, presenting an initial state of the simulation in which the ice giants (Uranus and Neptune) were in resonance with each other within a range of 15 AU from the Sun, and where the fifth giant planet was positioned between them and Saturn. The end result of this simulation, performed about 6,000 times by varying the total mass of the available planetesimals, offers an interesting solution: the simulated solar system, once exhausted the planetesimals and the orbital energy in excess, appears to be 10 times closer to the original if the fifth giant planet is actually expelled from the system itself.
Planet sighting
I am going to finish with a few words about some (nomad?) planet sighting, chosen not necessarily among the most recent, but rather among the most unusual ones. And I would also like to mention here a forthcoming article about the possibility that mankind, in a far away future, decides to establish space colonies or outposts in the Kuiper belt, the Oort Cloud and interstellar space.
* Object CFBDSIR2149 (Canada-France Brown Dwarfs Survey) is associated with a group of about thirty young stars known as AB Doradus Moving Group, that are moving through space together with the star AB Doradus. If there were evidence that the supposed nomad object actually belongs to the group, its age will then be between 50 and 120 million years, mass between 4 to 7 times that of Jupiter, and surface temperature about 700 K.
* HD106906b is not really a nomad planet; actually it orbits around a star. The strange thing here is its distance from the star – 650 UA (about 97 billion km). Quite a distance, if you think that Neptune orbits at an average distance of 30 AU from the Sun. There’s more: the planet is large and young, 11 times Jupiter’s mass and only of 13 million years of age, compared to 4.5 billion of the Earth. How could the planet have reach its current size in such a short time only having at its disposal the few resources of the protoplanetary disk at a similar distance? Several hypotheses have been proposed to explain this oddity, and the most likely is that HD106906 is in reality a binary system where the second star has not been able to ignite.
* The discovery of the dwarf planet 2006 SQ372 was made public only in 2008 during a specialised symposium held in Chicago. At that time the object was in the proximity of Neptune, following an elliptical orbit of the kind of Sedna, which would lead it to up to 150 billion km from the Sun. The celestial body could have been formed, like Pluto, in the Kuiper Belt, or it could more likely have come from the inner part of the Oort Cloud. These objects may in some cases be nomad planets, and it would be useful to study them more thoroughly whenever there the possibility arises. However, as far as 2006 SQ372 is concerned, this won’t be happening before 22,000 years from now.
References: Slow Boat to Centauri: a Millennial Journey Exploiting Resources Along the Way by Paul Gistler – JBIS vol. 66 – 2013 pp 302 , 31; Nomads of the Galaxy, by Louis E. Strigari et. al. – Kavli Institute for Particle Astrophysics and Cosmology, Stanford University; The Steppenwolf: a Proposal for a Habitable Planet in Interstellar Space by D.S. Abbot (Department of the Geophysical Sciences, University of Chicago) and E.R. Switzer (Kavli Institute for Cosmological Physics, University of Chicago; So many lonely planets with no star to guide them by Nadia Drake – Nature 2/12/10; HD106906b by Sabrina Pieragostini – Panorama 14/12/13; Pianeti nomadi, la Via Lattea ne è piena MEDIA INAF venerdì 24 febbraio 2012; A Gas Giant Ejected from our System? by Paul Gilster, Centauri Dreams, on November 11, 2011; An Icy Wanderer from the Oort Cloud by Paul Gilster, Centauri Dreams on August 18, 2008; Finding an Interstellar Wanderer by Paul Gilster,Centauri Dreams on May 17, 2011; New Findings on Rogue Planets by Paul Gilster,Centauri Dreams, on May 19, 2011.
The original version of this post in Italian, published on September 17, 2014, is available here.