Hot Neptunes Super Earths and Ocean Planets

Before the discovery of exoplanets and their diversity, when the Solar System was our sole reference for understanding planetary formation, there was a strong theoretical distinction between giant planets and terrestrial planets. A terrestrial planet was assumed to have formed between the star and the ice line, in a region where ices could not contribute towards the accretion of solid cores, and where only small planets of a few Earth masses, maximum, could form. Beyond the ice line, more massive cores of rocky material and ices could grow until they reached the critical mass above which a giant planet formed through rapid accretion of gas from the disk. In this scenario, the ice giants, Uranus and Neptune, remained aborted giant planets that formed from a disk that was already depleted in gas, because of the longer accretion times at their orbital distances (see Sect. 4.3.2.4).

This scenario has now been completely shattered by the diversity of objects that have been revealed by observation, and also by the now obvious role of planetary migration. Henceforth, we will have to take account of the fact that migration is not only necessary to understand the existence of giant exoplanets, but that it is also needed to understand the formation of Jupiter and Saturn (Alibert et al., 2005), and the architecture of the whole Solar System beyond Jupiter (Tsiganis et al., 2005 and

Sect. 4.3.2.6). Planetary migration within protoplanetary disks, which evolve and -

Fig. 7.27 (continued) Possible evolution of terrestrial spectral from 3900 million years ago to the present day. Based on a 'standard' model of the Earth's atmosphere, describing the evolution of the surface temperature and the atmospheric components (H2O, CO2, O2, O3, CH4), Kaltenegger et al. simulated the evolution of the apparent spectrum of our planet over the course of its history. The latter was broken down into 6 epochs before the present (BP): 3.9 x 109 BP (0); 3.5 x 109 BP (1); 2.4 x 109 BP (2); 2 x 109 BP (3); 2 x 109 BP (4); 0.8 x 109 BP (5). The 6 diagrams on the top show the visible spectrum (reflected light), while the 6 diagrams on the bottom show the infrared spectrum (thermal emission) (After Kaltenegger et al., 2007)

disperse rapidly (in a few million years for the gas and dust), may result in an astonishingly diverse range of planets, in terms of mass, chemical composition, and orbital distance. In addition, it may give rise to a population of planets with masses between those of giants and terrestrial planets. Such planets must be abundant, because radial-velocity detections of planets below 15 MEarth are accumulating despite the observational bias that does not favour low masses (Fig. 7.28). A striking example is the HD 69830 system, which contains three planets, whose minimum masses all lie between 10 and 18MEarth, and where one of them, the largest, orbits within the habitable zone. Among the thousands of models of planetary formation (including migration and the evolution of the disk) that were tested by Alibert et al. (2006) by varying the initial conditions, the one shown in Fig. 7.29 reproduces a system similar to the one that is observed. Here, migration is the factor that allows massive planets to be found within the inner region of the system. It is the progressive dissipation of the disk of gas and the initial density of solids that prevent these planets from evolving into giant planets. It may be noted that at the end of its formation, the

period (years)

Fig. 7.28 The mass and orbital period of known exoplanets. This diagram shows the approximately 209 currently known, detected either by radial-velocity or transit observations. For planets detected solely by radial velocity, the mass is the minimum mass (the inclination not being known). The straight lines indicate the amplitude of the variations in radial velocity produced by a star of 1 [email protected] (continuous line), 0.5 [email protected] (dashed lines), and [email protected] (dotted line). This gives an idea of the instrumental accuracy required for detection. Currently the HARPS instrument has a sensitivity better than 1 m/s, revealing a new population of planets (known as hot Neptunes or Super-Earths, depending on the authors), of less than 20MEarth (in violet). The fact that these detections have been made despite the strong observational bias favouring high masses demonstrates that these planets are very common. The violet circles mark the HD 69830 system (Fig. 7.29)

period (years)

Fig. 7.28 The mass and orbital period of known exoplanets. This diagram shows the approximately 209 currently known, detected either by radial-velocity or transit observations. For planets detected solely by radial velocity, the mass is the minimum mass (the inclination not being known). The straight lines indicate the amplitude of the variations in radial velocity produced by a star of 1 [email protected] (continuous line), 0.5 [email protected] (dashed lines), and [email protected] (dotted line). This gives an idea of the instrumental accuracy required for detection. Currently the HARPS instrument has a sensitivity better than 1 m/s, revealing a new population of planets (known as hot Neptunes or Super-Earths, depending on the authors), of less than 20MEarth (in violet). The fact that these detections have been made despite the strong observational bias favouring high masses demonstrates that these planets are very common. The violet circles mark the HD 69830 system (Fig. 7.29)

Fig. 7.29 The scenario for the formation of the triple system around HD 69830. The star HD 69830 has three planets, lying at distances of 0.0785, 0.186 and 0.63 AU, which were detected by radialvelocity measurements (Lovis et al., 2006). The minimum masses of these planets are 10.2, 11.8, and 18.1 Mg^, respectively. The curves shown in a thin line show the evolution of the mass of the solid core, while the thicker lines indicate the overall mass (After Alibert et al., 2006). For the 2 inner planets, the decrease in mass is caused by evaporation of the atmosphere through EUV irradiation

Fig. 7.29 The scenario for the formation of the triple system around HD 69830. The star HD 69830 has three planets, lying at distances of 0.0785, 0.186 and 0.63 AU, which were detected by radialvelocity measurements (Lovis et al., 2006). The minimum masses of these planets are 10.2, 11.8, and 18.1 Mg^, respectively. The curves shown in a thin line show the evolution of the mass of the solid core, while the thicker lines indicate the overall mass (After Alibert et al., 2006). For the 2 inner planets, the decrease in mass is caused by evaporation of the atmosphere through EUV irradiation third planet lay within the habitable zone, but, in this model the envelope of gas (H2 and He) would amount to about 8 MEarth. The layer of water that lies below this is therefore in the form of a super-critical fluid at a high temperature and pressure.

These 'intermediate' planets, which populate the short and medium periods may have various origins. The hottest may be giant planets greatly eroded by atmospheric loss caused by EUV radiation from the star (Baraffe et al., 2005). For the planets below the critical mass for the accretion of gas 10MEarth), they may be 'Super-Earths', formed between the ice line and the star, but having gained significant mass through the migrations of the embryo planet inwards in the system. It may also be that these planets consist of 50 per cent ice, having formed beyond the ice line and subsequently migrated inwards. The latter may have ended their migration in the habitable zone, and thus given rise to ocean planets (Leger et al., 2004; see Sect. 7.2.2). Knowing just the mass and periods of these objects is not sufficient to define their chemical composition, nor any possible atmosphere. Apart from their initial, variable amounts of different elements in their composition, degassing, photochemistry, capture of nebular gas, condensation, and escape all combine to give a vast, diverse range of possible atmospheric compositions. Theoretical studies of these 'new planets', of their atmospheres, and their spectral signatures are only just beginning.

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