“Floating” planets: questions from a journalist

1/ Do you think these free floating planets, detected by micro-lensing very far away from us, are the same objects than those you observed years ago in the Sigma Orionis cluster?

There are two options (for both types): either they have been formed in a similar way as stars (collapse and fragmentation of molecular clouds, with all their variations -effect of strong winds by massive, nearby stars, dynamical interactions due to close encounters, etc), or in a way similar to the planets in the Solar System (from the material left from the formation of the central star, which is organized as a disk during the first few million years of the star life).

We do not know how the IPMOs (Isolated Planetary Mass Objects) found in Sigma Ori and other regions (Trapezium, Lambda Orionis, etc) have been formed (catalog for confirmed objects). There are several hints, and they point out that they might have formed by the first mechanism (similar to stars). However, even if the majority are formed from collapse and fragmentation, some could be bona fide giant planets, expelled by a close encounter with another star or by the dynamical instabilities within the original planetary system (with other massive planets and/or the original disk surrounding the central star).

In any case, the detection of the new isolated objects by gravitational lensing does not say anything directly about how they were formed. The situation is very similar to the Sigma Ori IPMOs. So, they might be runaway planets or IPMOs formed in a stellar association which has dissolved after several hundred or thousand millions of years (after they have orbited several times around the center of the Galaxy).

However, it is true that  in the Sumi’s paper, the results indicate the formation mechanism might be different, based on the number of events. This might suggest they are formed as planets.

We have to take into account that the statistics is poor  in both cases, in the sense that the number of detected objects is so low that the error-bars are very important. So more observations are really needed.

So, they might be identical twins (from the perspective of the formation mechanism) or siblings (again, from the same perspective).

2/ What differs between the two types of objects?

Age. The SOri IPMOs are very young (3-5 Myr), whereas the new objects should be very old. In addition, although we do not know for sure, the composition might be different, since the universe is in continuous chemical evolution, being enriched of chemical elements heavier than hydrogen and helium (mainly by the death of massive stars).

In any case, a number of IPMOs identified in young stellar associations have been characterized with spectroscopy, a technique that gives us very interesting information about their nature and properties. The lest massive object identified so far ha a mass about 3 Mjupiter (the micro-lensing IPMOs have about 10 Mjupiter).

3/ Do you have more informations (since 1999) about S Ori 47?

We believe SOri 47 is a very low-mass brown dwarf, slightly above the deuterium burning limit (ie, some possible frontier between BD and IPMOs, based on the easiest of the nuclear reactions).

Planetary Mass Objects (PMO) have been found in different environments. In isolation but inside star forming regions, such as the IPMOs in SOri (ie, no nearby stars, no gravitational link between the object and the PMO), and close to low-mass stars or brown dwarfs.

An example of this last case is 2M1207, discovered by Gael Chavin and collaborators in 2004. A 5 Mjupiter PMO is located about 40 astronomical units from a 20-30 Mjupiter brown dwarf. The system has an age about 10 Myr. A more recent example is the case of T Cha, a low-mass star with a substellar object in formation, located in the middle of the gap of a circunstellar disk.

Eventually (but we do not know if this would happen), the low-mass companion (2M1207b) might be expelled and after thousand of million years, after it cools down, it would look like the objects discovered by gravitational lensing.

4/ Do you think that these free floating planets can be made alone, far away from any interstellar cloud or very young star forming region?

No, they should be formed in dense, rich environments such as the Orion Molecular Cloud and other similar to them. But the statistics are poor. A dozen objects is not significant. As a comparison, we estimate that the number of stars in the Galaxy is two hundred milliards (200,000,000,000).

5/ Do you think that these objects can be part of dark matter? If yes, what percentage?

Indeed, but just an insignificant fraction, not relevant at all. Probably, they do not have any impact on the general gravitational field of the Galaxy. So, they should not be the solution for the dark matter problem.

6/ Please, add any comment you find important about this work!

It quite interesting to apply a different technique and confirm that the dark, empty interstellar space is not so empty after all. On top of this, the mass is estimated more directly (whereas we have to apply theoretical models for the IPMOs found by photometric searches, such as those in Sigma Ori).

However, it is true that this is not the first finding of the type (very low-mass, extremely cool objects). Similar objects have been found in very young stellar associations, just after they have been formed. And more massive siblings, called ultra cool dwarfs, are being discovered in isolation. These objects are also very old, but with masses about 20-70 Mjupiter. They are temperatures are hotter, reaching about 400-600 K. What it is relevant is that we are starting to realize that the Galaxy is filled with a plethora of strange objects which reassemble the Solar System giant planets, but not quite. The new all-sky, deep surveys in the near- and mid-infrared, which are being conducted now, will uncover a significant number of them, even very close to the Solar System (even closer than the nearest star), and new instrumentation, such as the E-ELT, will be used to study their properties.

In any case, the frequency of events, its dependence on the event time scale (how long they last) and its relation  with the mass of the objects suggest a different origin (stars and brown dwarfs on one hand, micro-lensing IPMO son the other). The possibility of having runaway planets is, indeed, fascinating.