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No large population of unbound or wide-orbit Jupiter-mass planets

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Abstract

Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes1,2,3. Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form4. A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions5,6 as well as wide-field surveys7, but these studies are incomplete8,9,10 for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis11 of 474 microlensing events found an excess of ten very short events (1–2 days)—more than known stellar populations would suggest—indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories3,12 and surveys of young clusters8,9,10. Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010–15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1–2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories3,12.

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Figure 1: Observed distribution of timescales of 2,617 high-quality microlensing events discovered by OGLE in 2010–15.
Figure 2: Distribution of event timescales corrected for the detection efficiency.
Figure 3: Light curves of ultrashort microlensing event candidates.

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Acknowledgements

We thank M. Kubiak and G. Pietrzyński, former members of the OGLE team, for their contribution to the collection of the OGLE photometric data over the past years. The OGLE project has received funding from the National Science Center, Poland through grant MAESTRO 2014/14/A/ST9/00121 to A.U.

Author information

Affiliations

Authors

Contributions

P.M. analysed and interpreted the data, and prepared the manuscript. A.U. initiated the project, reduced the data, and conducted detection efficiency simulations. All authors collected the OGLE photometric observations, reviewed, discussed and commented on the present results and on the manuscript.

Corresponding author

Correspondence to Przemek Mróz.

Additional information

Reviewer Information Nature thanks C. Clanton, S. Raymond and T. Sumi for their contribution to the peer review of this work.

Publisher's note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Figure 1 Galactic bulge luminosity function used for simulations.

a, Deep luminosity function (LF) for subfield BLG513.12, which was observed both by the OGLE-IV survey and by the Hubble Space Telescope (HST)38. Both luminosity functions overlap in the range 16 mag < I < 18 mag. This deep luminosity function was used as a template to generate artificial microlensing events in analysed fields, after shifting to match the centroid of the red clump giant stars in a given field. b, Comparison between the observed luminosity function for subfield BLG512.32 and the simulated luminosity function.

Extended Data Figure 2 Detection efficiency curves.

Detection efficiencies as a function of the Einstein timescale tE for all analysed fields (averages for all subfields in the given field). Fields BLG501, BLG505 and BLG512 were observed with a 20-min cadence, and the remaining fields with a 60-min cadence. Error bars are the 1σ Poisson uncertainties on the counts of the number of simulated events in a given tE bin.

Extended Data Figure 3 Comparison between measured Einstein timescales tE,out and ‘real’ (simulated) timescales tE,in for simulated events.

Only events passing selection criteria from Extended Data Table 3 (including the cut on the blending parameter fs > 0.1) are shown. Note that the colour scale is logarithmic. There is no systematic offset between measured and real timescales.

Extended Data Figure 4 Comparison between measured and ‘real’ (simulated) parameters.

a, Ratio between the measured Einstein timescale tE,out and ‘real’ (simulated) timescale tE,in for simulated events versus the blending parameter fs = Fs/(Fs + Fb). Timescales of faint and highly blended (fs < 0.1) events are not well measured and are biased by a strong degeneration between Einstein timescale, blending and impact parameters. Timescales of events showing a high negative blending (fs > 1.5) are systematically underestimated, but the bias is relatively small and such events comprise a negligible fraction of all events. b, Distributions of tE,out/tE,in for simulated events passing selection criteria from Extended Data Table 3 (including the cut on the blending parameter fs > 0.1). Regardless of the timescale, there is no systematic bias between measured and real timescales within 1%. For 90% of simulated events 0.63 < tE,out/tE,in < 1.65. The MAD is the median absolute deviation from the data’s median.

Extended Data Figure 5 Constraints on IMF slopes.

a, Assuming that all lenses are single; b, assuming binary fraction fbin = 0.4.

Extended Data Table 1 Best-fitting parameters for ultrashort microlensing event candidates
Extended Data Table 2 Basic information about analysed fields
Extended Data Table 3 Selection criteria for high-quality microlensing events
Extended Data Table 4 Number of events detected in individual timescale bins
Extended Data Table 5 Detection efficiencies for the analysed fields

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Mróz, P., Udalski, A., Skowron, J. et al. No large population of unbound or wide-orbit Jupiter-mass planets. Nature 548, 183–186 (2017). https://doi.org/10.1038/nature23276

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