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Nomadic Exoplanets and the Nasa Strategic Vision for 2050 T Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8169.pdf NOMADIC EXOPLANETS AND THE NASA STRATEGIC VISION FOR 2050 T. Marshall Eubanks1, 1Asteroid Initiatives LLC, Clifton, VA 20124 USA; [email protected]; Mass / MassEarth Introduction: NASA’s strategic goals include the 0.01 0.1 1 10 100 1000 7 search for planets around other stars, the characteriza- Expected Minimum Distance 4*105 tion of their properties, and the identification of exo- 6 planets that could possibly harbor life. In addition, with 5 the discovery of Proxima b, an exoplanet orbiting in the 3*105 Proxima Centauri habitable zone of the star Proxima Centauri, the clos- 4 est star to the Sun, long range planning is beginning to Saturn 2*105 consider its possible in situ exploration by spacecraft. 3 Neptune Jupiter Earth Distance (AU) These strategic goals should be extended to include no- 2 Oort Cloud 1*105 madic (or rogue) exoplanets, planets not orbiting any Distance to Nearest Object (ly) star. 1 Moon While Proxima b will remain the closest exoplanet or- 0 0.0001 0.001 0.01 0.1 1 10 biting a star, microlensing surveys indicate that there Mass / MassJupiter are likely to be closer nomadic planets [1]. To date, discovered nomads have been mostly either distant ob- jects found through microlensing, or young, warm, no- Figure 1: The expected minimum distance, Rmin, as a mads found near star formation regions. However, function of nomadic planet mass, based on microlens- there should be significant numbers of mature nomadic ing power law number-density models [1]. Although exoplanets close enough to be discovered with exist- the uncertainties are fairly large, the nearest nomadic ing or future astronomical resources, including possi- planets are expected to be as close or closer than Prox- bly dozens of planets closer to us than Proxima b. Al- ima Centauri for a wide range of masses. The estimated though mature nomads will appear to be very cold astro- extent of the Oort cloud and the distance to Proxima nomically, superEarth nomads can retain heat, be Ocean Centauri are shown as horizontal lines. Worlds and conceivably support exobiologies [2, 3]. Nearby nomadic planets are thus extremely relevant the expected distance of the nearest “dark-Jupiter” be- to the Origins, Workings and Life goals of the NASA ing ∼77% of the distance to Proxima Centauri. Sumi strategic vision for 2050. Finding the closest nomadic et al. [4] also provide a power law model for nomadic exoplanets should become an important part of NASAs planet number density as a function of mass. Figure 1 strategic goals, particularly the exoplanets closer, and shows the expected minimum distances, Rmin and these thus easier to reach, than Proxima b. In order to fa- uncertainties as a function of nomad mass [1]. It is nec- cilitate the search for nomadic planets, NASA should essary to extrapolate the power law density models for support a large far-IR (100 µm wavelength) space tele- masses ≪ the mass of Jupiter [6], leading to a factor of scope and support planet searches with long wavelength almost 6 uncertainty in Rmin for Earth-mass nomads. (1 - 10 meters) radio arrays. Nomadic planet number Reducing the uncertainty in the nomadic planet number statistics remain very uncertain for sub-Jupiter masses, density function at lower masses is essential for better and should also be improved through support of high modeling of Rmin for Earth mass planets. The planned cadence microlensing surveys. WFIRST telescope should be able to detect and char- The Expected Distance to the Nearest Nomadic acterize the population of nomadic superEarths in the Planets: Gravitational microlensing surveys have Galactic bulge with microlensing [7]; it is important that shown that Jupiter-mass nomads are more populous NASA support microlensing surveys by this or a com- than main sequence stars. Sumi et al. [4] estimated the parable space telescope. ratio of the number density of Jupiter-mass unbound ex- Finding Nearby Nomadic Exoplanets: Figure 2 oplanets, nJ , and the number density of main sequence shows the black body flux density expected from a set of +1.3 stars n⋆, with nJ / n⋆ = 1.9−0.8 from microlensing data. hypothetical planets, matching the Earth, Uranus, Nep- The stellar number density is well known near the Sun tune, Saturn and Jupiter in mass, radius and internal heat [5], yielding an estimate for nJ [1] of flux, with each assumed to be at Rmin for a body of its +6.4 −3 −3 mass. A super-Jupiter with 10 times the mass of Jupiter nJ = (6.7−3 0) × 10 ly (1) . is included based on a heat flux scaling model [1]. Fig- and thus an estimate for the expected mean distance to ure 2 also shows flux density limits for the ALMA [8], the nearest Jupiter mass nomadic planet, Rmin, of cooled WISE [9], cooled Spitzer [10]), SPICA [11] and +0.7 JWST [10] instruments. Existing instruments should Rmin(MJupiter) = 3.28−0.6 ly , (2) Planetary Science Vision 2050 Workshop 2017 (LPI Contrib. No. 1989) 8169.pdf Wavelength (µm) 1000 100 10 Astrobiologies on Nomadic Exoplanets: Nomadic 173 K ’’super−Jupiter’’ planets could be ocean worlds, with insulated oceans 10000 100 K ’’Jupiter’’ 80 K ’’Saturn’’ 53 K ’’Neptune’’ surviving with no stellar heat input [1]. Stevenson [2] 1000 29 K ’’Uranus’’ 36 K ’’Earth’’ WISE proposed that Earth-mass planets could have surface ALMA (1 hr) 100 oceans of liquid water, and thus conceivably biologies, Jy) µ SPICA with radioactive heat being retained by thick Hydrogen- 10 Spitzer Flux ( ALMA (24 hr) Helium (H-He) atmospheres with pressure induced far- JWST 1 IR opacity. The discovery that for M & 4 MEarth terres- ∝ 0.1 trial planet radii are roughly mass strongly suggests that H-He atmospheres are common for at least these 0.01 1000 10000 100000 super-Earths [21, 22]. Nomadic “Steppenwolf” plan- Frequency (GHz) ets, with M & 3.5 MEarth, could instead have internal liquid water oceans insulated by a thick shell of ice [3]. Figure 2: The IR flux density for black bodies with the There are of course a number of candidate ocean worlds, same radius and internal power generation as the ac- warmed by tidal heating, in the Solar System [23]; sim- tual Earth, Uranus, Neptune, Saturn and Jupiter, plus ilarly tidally-heated oceans could exist on nomadic exo- a model-derived “super-Jupiter” with a mass of 10 moons [24]. The exploration of nearby nomadic planets MJupiter, each modeled as a black body at their ex- thus has the potential to both benefit from and inform pected Rmin [1], together with flux density limits for the NASA effort for the exploration of the biological various actual (ALMA, cooled Spitzer, cooled WISE) potential of Ocean Worlds in our solar system. and planned (SPICA and JWST) telescopes and arrays. References: [1] T. M. Eubanks (2015) Nomadic Planets Near the Solar System accepted by Planetary and Space be able to detect nearby nomadic gas-giants, while de- Science. [2] D. J. Stevenson (1999) Nature 400:32 doi. tection of nearby nomadic Earths and superEarths will [3] D. S. Abbot, et al. (2011) Ap J Lett 735:L27 doi. arXiv:1102.1108. [4] T. Sumi, et al. (2011) Nature likely require surveys by a new generation of space 473:349 doi.arXiv:1105.3544. [5] G. Chabrier (2001) telescopes, such as the Far-Infrared Surveyor Mission Ap J 554:1274 doi.arXiv:astro-ph/0107018. (FIRS) [12] currently under consideration. [6] L. E. Strigari, et al. (2012) Mon Not RAS 423:1856 doi. A different means of discovering nearby magnetized arXiv:1201.2687. [7] C. B. Henderson, et al. (2016) planets is through the detection of their non-thermal Astron J 152:96 doi.arXiv:1603.05249. [8] A. Baudry radio emissions. The strongly magnetized bodies in (2008) in 2nd MCCT-SKADS Training School. Radio the solar system (the Earth plus the 4 giant planets) Astronomy: Fundamentals and the New Instruments. [9] E. L. all exhibit strong non-thermal radio emissions driven Wright, et al. (2010) Astron J 140:1868 doi. by the electron Cyclotron Maser Instability (CMI) [13]. arXiv:1008.0031. [10] M. J. Barlow (2012) in IAU CMI emissions are generated by celestial bodies mov- Symposium vol. 283 of IAU Symposium 295–301 doi. ing through a plasma, with either the body or the [11] T. Onaka, et al. (2004) in 5th International Conference on Space Optics (Edited by B. Warmbein) vol. 554 of ESA plasma, or both, possessing a significant magnetic field Special Publication 297–302. [12] M. Meixner, et al. (2016) [14, 15, 16], or even from the rapid rotation of a magne- in Society of Photo-Optical Instrumentation Engineers tized body [17]. Such emissions provide a non-thermal (SPIE) Conference Series vol. 9904 of Proceedings of the means of detecting magnetized exoplanets [18], includ- SPIE 99040K doi.arXiv:1608.03909. [13] P. Zarka ing magnetized nomads [19, 1]. In the solar system, (2011) Planetary, Solar and Heliospheric Radio Emissions Jupiter produces a very strong “unipolar” CMI radio (PRE VII) 287–301. [14] P. Zarka (1998) J Geophys Res flux, primarily due to electrons flowing through the 103:20159 doi. [15] J.-M. Grießmeier, et al. (2011) Radio Jupiter-Io flux tube. A Jupiter-Io analogue at the ex- Science 46:RS0F09 doi. [16] B. Cecconi, et al. (2012) pected distance of the nearest Jupiter-mass exoplanet Planet Space Sci 61:32 doi. [17] J. D. Nichols, et al. (2012) (see Equation 2) would have a maximum flux of ∼10 Ap J 760:59 doi.arXiv:1210.1864.
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