Terrestrial Planet Zone Research Articles

Early terrestrial planet formation by torque-driven convergent migration of planetary embryos

The massive cores of the giant planets are thought to have formed in a gas disk by the accretion of pebble-sized particles whose accretional cross-section was enhanced by aerodynamic gas drag1,2. A commonly held view is that the terrestrial planet system formed later (30–200 Myr after the dispersal of the gas disk) by giant collisions of tens of lunar- to Mars-sized protoplanets3,4. Here we propose, instead, that the terrestrial planets of the Solar System formed earlier by the gas-driven convergent migration of protoplanets towards ~1 au. To investigate situations in which convergent migration occurs and determine the thermal structure of the gas and pebble disks in the terrestrial planet zone, we developed a radiation–hydrodynamic model with realistic opacities5,6. We find that protoplanets grow in the first 10 Myr by mutual collisions and pebble accretion, and gain orbital eccentricities by gravitational scattering and the hot-trail effect7,8. The orbital structure of the inner Solar System is well reproduced in our simulations, including its tight mass concentration at 0.7–1 au and the small sizes of Mercury and Mars. The early-stage protosolar disk temperature exceeds 1,500 K inside 0.4 au, implying that Mercury grew in a highly reducing environment next to the evaporation lines of iron and silicates, influencing Mercury’s bulk composition9. A dissipating gas disk, however, is cold, and pebbles drifting from larger heliocentric distances would deliver volatile elements. A full 2D radiation–hydrodynamic model of a protoplanetary disk shows that rocky planets can be formed early, and not tens of million years after the dispersal of the gas disk as usually assumed, by means of gas-driven migration of planetesimals around 1 au. The model reproduces well the structure of the inner Solar System.

Linking planetary embryo formation to planetesimal formation

Context. The growth-timescales of planetary embryos and their formation process are imperative for our understanding on how planetary systems form and develop. They determine the subsequent growth mechanisms during the life stages of a circumstellar disk. Aims. We quantify the timescales and spatial distribution of planetary embryos through collisional growth and fragmentation of dynamically forming 100 km sized planetesimals. In our study, the formation timescales of viscous disk evolution and planetesimal formation are linked to the formation of planetary embryos in the terrestrial planet zone. Methods. We connected a one-dimensional model for viscous gas evolution, dust and pebble dynamics, and pebble flux-regulated planetesimal formation to the N-body code LIPAD. Our framework enabled us to study the formation, growth, fragmentation, and evolution of planetesimals with an initial size of 100 km in diameter for the first million years of a viscous disk. Results. Our study shows the effect of the planetesimal surface density evolution on the preferential location and timescales of planetary embryo formation. Only the innermost embryos (<2 au) in our study form well within the lifetime of an active pebble flux for any disk studied. Higher planetesimal disk masses and steeper planetesimal surface density profiles result in more massive embryos within a larger area, rather than in a higher number of embryos. A one-dimensional analytically derived model for embryo formation based on the local planetesimal surface density evolution is presented. This model can reproduce the spatial distribution, formation rate, and total number of planetary embryos at a fraction of the computational cost of the N-body simulations. Conclusions. The formation of planetary embryos in the terrestrial planet zone occurs simultaneously with the formation of planetesimals. The local planetesimal surface density evolution and the orbital spacing of planetary embryos in the oligarchic regime are good constraints for modeling planetary embryo formation analytically. Our embryo formation model is a valuable asset in future studies of planet formation.

The Unique Comet Impact Pattern with Reference to Southern Tibet

In the terrestrial planet zone, Comets start outgassing due to solar radiation. This can make comet composition fragile enough to break under the gravitational gradient. Examples like those of Comet Shoemaker-Levy 9 [1] show that it is not unusual for comets to disintegrate due to gravitational gradients. In the event of an impact of such a disintegrated comet on earth, multiple coherent impact craters will be distributed over a large area. The low-density porous composition of the comet will result in the formation of “large flat-floored craters” spread over a large area with a network of faults and fractures as described by Roddy and co-workers (Roddy, 1976; Roddy et al., 1977) [2] [3]. Due to its unusual appearance these impacts patterns are different than rock/metal asteroid impact craters. As a result, these sites are difficult to identify. There are multiple such sites on the planet earth which are under study for impact but due to complex nature of comet impact craters, they have been a conundrum for some time. One such possible site matching the comet impact features can be observed in the southern Tibet between the latitudes of 82°30'E and 90°30'E and 29°N and 33°N. This study indicates that the lakes in this region were formed by the impact of a single disintegrating comet that hit the region. Observation indicates that the lakes are unusually closely aligned and have a steep slope facing circular feature on the eastern side. Fractures and faults connecting these lakes can be observed over entire subject site hinting towards the multiple impact craters. These craters are large flat-floored and spread over a large area. Gravitational anomaly in the lake region along with the presence of Ultra High Pressure (UHP) minerals like cubic Born Nitride, micro-diamond with the inclusion of Platinum Group Elements (PGE), coesite, Stishovite, osbornite and other complex finds in the region support the possibility of impact.

Large impacts around a solar-analog star in the era of terrestrial planet formation.

The final assembly of terrestrial planets occurs via massive collisions, which can launch copious clouds of dust that are warmed by the star and glow in the infrared. We report the real-time detection of a debris-producing impact in the terrestrial planet zone around a 35-million-year-old solar-analog star. We observed a substantial brightening of the debris disk at a wavelength of 3 to 5 micrometers, followed by a decay over a year, with quasi-periodic modulations of the disk flux. The behavior is consistent with the occurrence of a violent impact that produced vapor out of which a thick cloud of silicate spherules condensed that were then ground into dust by collisions. These results demonstrate how the time domain can become a new dimension for the study of terrestrial planet formation.

COPIOUS AMOUNTS OF HOT AND COLD DUST ORBITING THE MAIN SEQUENCE A-TYPE STARS HD 131488 AND HD 121191

We report two new dramatically dusty main sequence stars: HD 131488 (A1V) and HD 121191 (A8V). HD 131488 is found to have substantial amounts of dust in its terrestrial planet zone (L_IR/L_bol~4x10^-3), cooler dust further out in its planetary system, and an unusual mid-infrared spectral feature. HD 121191 shows terrestrial planet zone dust (L_IR/L_bol~2.3x10^-3), hints of cooler dust, and shares the unusual mid-infrared spectral shape identified in HD 131488. These two stars belong to sub-groups of the Scorpius-Centaurus OB association and have ages of ~10 Myr. HD 131488 and HD 121191 are the dustiest main sequence A-type stars currently known. Early-type stars that host substantial inner planetary system dust are thus far found only within the age range of 5-20 Myr.

THE NEARBY, YOUNG, ISOLATED, DUSTY STAR HD 166191

We report an in-depth study of the F8-type star HD 166191, identified in an ongoing survey for stars exhibiting infrared emission above their expected photospheres in the Wide-field Infrared Survey Explorer all-sky catalog. The fractional IR luminosity measured from 3.5 to 70 $\mu$m is exceptionally high (L$_{IR}$/L$_{bol}$ $\sim$10%). Near-diffraction limited imaging observations with the T-ReCS Si filter set on the Gemini South telescope and adaptive optics imaging with the NIRC2 Lp filter on the Keck II telescope confirmed that the excess emission coincides with the star. Si-band images show a strong solid-state emission feature at $\sim$10 $\mu$m. Theoretical evolutionary isochrones and optical spectroscopic observations indicate a stellar age in the range 10-100 Myr. The large dust mass seen in HD 166191's terrestrial planet zone is indicative of a recent collision between planetary embryos or massive ongoing collisional grinding associated with planet building.

ASPITZERMIPS STUDY OF 2.5-2.0M☉STARS IN SCORPIUS-CENTAURUS

We have obtained Spitzer Space Telescope Multiband Imaging Photometer for Spitzer (MIPS) 24 {\mu}m and 70 {\mu}m observations of 215 nearby, Hipparcos B- and A-type common proper motion single and binary systems in the nearest OB association, Scorpius-Centaurus. Combining our MIPS observations with those of other ScoCen stars in the literature, we estimate 24 {\mu}m B+A-type disk fractions of 17/67 (25+6%), 36/131 (27+4%), and 23/95 (24+5%) for Upper Scorpius (\sim11 Myr), Upper Centaurus Lupus (\sim15 Myr), and Lower Centaurus Crux (\sim17 Myr), respectively, somewhat smaller disk fractions than previously obtained for F- and G-type members. We confirm previous IRAS excess detections and present new discoveries of 51 protoplanetary and debris disk systems, with fractional infrared luminosities ranging from LIR/L\ast = 1e-6 to 1e-2 and grain temperatures ranging from Tgr = 40 - 300 K. In addition, we confirm that the 24 {\mu}m and 70 {\mu}m excesses (or fractional infrared luminosities) around B+A type stars are smaller than those measured toward F+G type stars and hypothesize that the observed disk property dependence on stellar mass may be the result of a higher stellar companion fraction around B- and A-type stars at 10 - 200 AU and/or the presence of Jupiter-mass companions in the disks around F- and G- type stars. Finally, we note that the majority of the ScoCen 24 {\mu}m excess sources also possess 12 {\mu}m excess, indicating that Earth-like planets may be forming via collisions in the terrestrial planet zone at \sim10 - 100 Myr.

THE AGE OF THE HD 15407 SYSTEM AND THE EPOCH OF FINAL CATASTROPHIC MASS ACCRETION ONTO TERRESTRIAL PLANETS AROUND SUN-LIKE STARS

From optical spectroscopic measurements we determine that the HD 15407 binary system is ~80 Myr old. The primary, HD 15407A (spectral type F5V), exhibits strong mid-infrared excess emission indicative of a recent catastrophic collision between rocky planetary embryos or planets in its inner planetary system. Synthesis of all known stars with large quantities of dust in their terrestrial planet zone indicates that for stars of roughly Solar mass this warm dust phenomenon occurs at ages between 30 and 100 Myr. In contrast, for stars of a few Solar masses, the dominant era of the final assembling of rocky planets occurs earlier, between 10 and 30 Myr age. The incidence of the warm dust phenomenon, when compared against models for the formation of rocky terrestrial-like bodies, implies that rocky planet formation in the terrestrial planet zone around Sun-like stars is common.

IPHAS A-type Stars with Mid-IR Excesses in Spitzer Surveys

AbstractThe Isaac Newton Photometric H-Alpha Survey (IPHAS) provides (r′-Hα)-(r′-i′) colors, which can be used to select AV0-5 Main Sequence star candidates (age~20-200 Myr). By combining a sample of 23050 IPHAS-selected A-type stars with 2MASS, GLIMPSE and MIPSGAL photometry we searched for mid-infrared excesses attributable to dusty circumstellar disks. Positional cross-correlation yielded a sample of 2692 A-type stars, of which 0.6% were found to have 8-μm excesses above the expected photospheric values. The low fraction of main sequence stars with mid-IR excesses found in this work indicates that dust disks in the terrestrial planet zone of Main Sequence intermediate mass stars are rare. Dissipation mechanisms such as photo-evaporation, grain growth, collisional grinding or planet formation could possibly explain the depletion of dust detected in the inner regions of these disks.

Warm Dust in the Terrestrial Planet Zone of a Sun‐like Pleiades Star: Collisions between Planetary Embryos?

Only a few solar-type main-sequence stars are known to be orbited by warm dust particles; the most extreme is the G0 field star BD +20 307 that emits ~4% of its energy at mid-infrared wavelengths. We report the identification of a similarly dusty star HD 23514, an F6-type member of the Pleiades. A strong mid-IR silicate emission feature indicates the presence of small warm dust particles, but with the primary flux density peak at the nonstandard wavelength of ~9 μm. The existence of so much dust within an AU or so of these stars is not easily accounted for given the very brief lifetime in orbit of small particles. The apparent absence of very hot (≳1000 K) dust at both stars suggests the possible presence of a planet closer to the stars than the dust. The observed frequency of the BD +20 307/HD 23514 phenomenon indicates that the mass equivalent of Earth's Moon must be converted, via collisions of massive bodies, to tiny dust particles that find their way to the terrestrial planet zone during the first few hundred million years of the life of many (most?) Sun-like stars. Identification of these two dusty systems among youthful nearby solar-type stars suggests that terrestrial planet formation is common.

Relationship of comets and planets

We analyze our earlier data on the numerical integration of the equations of motion for 274 short-period comets (with the period P<200 yr) on a time interval of 6000 yr. As many as 54 comets had no close approaches to planets, 13 comets passed through the Saturnian sphere of action, and one comet passed through the Uranian sphere of action. The orbital elements of these 68 comets changed by no more than ±3 percent in a space of 6000 yr. As many as 206 comets passed close to Jupiter. We confirm Everhart’s conclusion that Jupiter can capture long-period comets with q = 4–6 AU and i < 9° into short-period orbits. We show that nearly parabolic comets cross the solar system mainly in the zone of terrestrial planets. No relationship of nearly parabolic comets and terrestrial planets was found for the epoch of the latest apparition of comets. Guliev’s conjecture about two trans-Plutonian planets is based on the illusory excess of cometary nodes at large heliocentric distances. The existence of cometary nodes at the solar system periphery turns out to be a solely geometrical effect.

Exotic Earths: Forming Habitable Worlds with Giant Planet Migration

Close-in giant planets (e.g., "hot Jupiters") are thought to form far from their host stars and migrate inward, through the terrestrial planet zone, via torques with a massive gaseous disk. Here we simulate terrestrial planet growth during and after giant planet migration. Several-Earth-mass planets also form interior to the migrating jovian planet, analogous to recently discovered "hot Earths." Very-water-rich, Earth-mass planets form from surviving material outside the giant planet's orbit, often in the habitable zone and with low orbital eccentricities. More than a third of the known systems of giant planets may harbor Earth-like planets.

On the possibility of terrestrial planet formation in hot-Jupiter systems

About a fifth of the exoplanetary systems that have been discovered contain a so-called hot-Jupiter – a giant planet orbiting within 0.1 AU of the central star. Since these stars are typically of the F/G spectral type, the orbits of any terrestrial planets in their habitable zones at ~1 AU should be dynamically stable. However, because hot-Jupiters are thought to have formed in the outer regions of a protoplanetary disc, and to have then migrated through the terrestrial planet zone to their final location, it is uncertain whether terrestrial planets can actually grow and be retained in these systems. In this paper we review attempts to answer this question. Initial speculations, based on the assumption that migrating giant planets will clear planet-forming material from their swept zone, all concluded that hot-Jupiter systems should lack terrestrial planets. We show that this assumption may be incorrect, for when terrestrial planet formation and giant planet migration are simulated simultaneously, abundant solid material is predicted to remain from which terrestrial planet growth can resume.

Mid?Infrared Observations of T Tauri Stars: Probing the Star?Disk Connection in Rotational Evolution

We present mid-IR N-band (λeff = 10.2 μm) photometry of a carefully selected sample of T Tauri stars thought to be single from the Taurus-Auriga molecular cloud. Infrared excesses in these stars are generally attributed to circumstellar dust disks. Combining observations at 2.16 (Ks band) and 10.2 μm (N band) we probe a region in the circumstellar dust disk from a few stellar radii through the terrestrial planet zone (0.02-1.0 AU). By analyzing the distribution of the (Ks - N) color index with respect to previously measured photometric rotation periods we investigate what role circumstellar disks play in the rotational evolution of the central star. The resulting positive correlation between these two variables is consistent with the notion that a star-disk interaction facilitates the regulation of angular momentum during the T Tauri stage. We also demonstrate how including nonsingle stars in such an analysis weakens any correlation in the relation between (Ks - N) color and period. To further understand disk properties we also present spectral energy distributions for a few objects with new ground-based M-band (λeff = 4.8 μm) and Q-band (λeff = 20 μm) data and compare them to a geometrically thin, optically thick disk model.

Constraining the Lifetime of Circumstellar Disks in the Terrestrial Planet Zone: A Mid‐Infrared Survey of the 30 Myr old Tucana‐Horologium Association

We have conducted an N-band survey of 14 young stars in the ~30 Myr old Tucana-Horologium association to search for evidence of warm, circumstellar dust disks. Using the MIRAC-BLINC camera on the Magellan I (Baade) 6.5 m telescope, we find that none of the stars have a statistically significant N-band excess compared to the predicted stellar photospheric flux. Using three different sets of assumptions, this null result rules out the existence of the following around these post-T Tauri stars: (1) optically thick disks with inner hole radii of ≲0.1 AU, (2) optically thin disks with masses of less than 10-6 M⊕ (in ~1 μm sized grains) within ≲10 AU of these stars, and (3) scaled-up analogs of the solar system zodiacal dust cloud with more than 4000 times the emitting area. Our survey was sensitive to dust disks in the terrestrial planet zone with fractional luminosity of log(Ldust/L*) ~ 10-2.9, yet none were found. Combined with results from previous surveys, these data suggest that circumstellar dust disks become so optically thin as to be undetectable at N band before age ~20 Myr. We also present N-band photometry for several members of other young associations and a subsample of targets that will be observed with the Spitzer Space Telescope by the Formation and Evolution of Planetary Systems Legacy Science Program. Finally, we present an absolute calibration of MIRAC-BLINC for four filters (L, N, 11.6, and Qs) on the Cohen-Walker-Witteborn system.

Formation of Planetesimals and Accretion of the Terrestrial Planets

Planetesimals formed in the solar nebula by collisional coagulation. Dust aggregates settled toward the central plane, the larger ones growing by sweeping up smaller ones. A thin, dense layer of particles formed; shear-generated turbulence and differential motions induced by gas drag inhibited gravitational instability. Growth proceeded by collisions, producing planetesimals on a timescale of a few thousand years in the terrestrial zone. For bodies smaller than about a kilometer, motions were dominated by gas drag, and impact velocities decreased with size. At larger sizes gravitational interactions became significant, and velocities increased due to mutual perturbations. Larger bodies then grew more rapidly, this “runaway” led to formation of tens to hundreds of lunar-to Mars-sized planetary embryos in the zone of terrestrial planets. The final accretion of these bodies into a few planets involved large impacts, and occurred on a timescale of 107 to 108 years. This scenario gives a reasonably consistent picture of the origin of the terrestrial planets, but does not account for the anomalously low eccentricities of the Earth and Venus.