9. 暴KOKUBO 走的AND 成IDA
長の様子
最大の天体
平均値
20 KOKUBO AND IDA
FIG. 4. Time evolution of the maximum mass (solid curve) and the mean
微惑星の暴走的成長
→ 原始惑星が誕生する
mass (dashed curve) of the system.
than this range are not statistically valid since each mass bin often
has only a few bodies. First, the distribution tends to relax to a
decreasing function of mass through dynamical friction among
FIG. 3. Snapshots of a planetesimal system on the a–e plane. The circles
represent planetesimals and their radii are proportional to the radii of planetesi-mals.
The system initially consists of 3000 equal-mass (1023 g) planetesimals.
FIG. 4. Time evolution of the maximum mass (solid curve) and the mean
mass (dashed curve) of the system.
than this range are not statistically valid since each mass bin often
has only a few bodies. First, the distribution tends to relax to a
decreasing function of mass through dynamical friction among
(energy equipartition of) bodies (t =50,000, 100,000 years).
Second, the distributions tend to flatten (t =200,000 years). This
is because as a runaway body grows, the system is mainly heated
by the runaway body (Ida and Makino 1993). In this case, the
eccentricity and inclination of planetesimals are scaled by the
軌道長半径 [AU] 軌道離心率
質量 [1023g]
時間 [年]
[Kokubo Ida, 2000]
11. ジャイアントインパクト
1134 KOKUBO, KOMINAMI, 軌道長半径 [AU]
長い時間をかけて原始惑星同士の軌道が乱れる
→ 互いに衝突・合体してより大きな天体に成長
軌道離心率
2.—Snapshots of the system on the a-e (left) and a-i (right) planes at t ¼ 0, 106, are proportional to the physical sizes of the planets.
planets is hnMi ’ 2:0 ! 0:6, whichmeans that the typical result-ing
system consists of two Earth-sized planets and a smaller
planet. In thismodel,we obtain hnai ’ 1:8 ! 0:7. In other words,
one or two planets tend to form outside the initial distribution of
protoplanets. In most runs, these planets are smaller scattered
planets. Thus we obtain a high efficiency of h fai ¼ 0:79 ! 0:15.
The accretion timescale is hTacci ¼ ð1:05 ! 0:58Þ ; 108 yr. These
results are consistent with Agnor et al. (1999), whose initial con-ditions
are the same as the standard model except for !¼ 8.
Fig. [Kokubo Ida, 2006]
21. random velocity of planetesimals is pumped up as high as
the escape velocity of protoplanets. This high random veloc-ity
On the other in circular orbits HD 192263 with 多様なmakes 円盤the かaccretion らprocess 生まslow and inefficient and thus
Tgrow longer. This accretion inefficiency れるis a 多severe 様problem
な惑星
!1e100 for in situ formation case. It is difficult slingshot model circular orbits the magnetic may be weak disks may be Terrestrial Jovian planets planetary accretion, key process systems.
We confirmed holds in !solid ¼ !1ða=! ¼ 1=2; 3=We derived systems depend disk profile growth timescale and (17), respectively, a
Mdisk T c o n tTdisk Tg ro w Tdisk
原始惑星系円盤の質量
軌道長半径 (中心星からの距離)
Fig. 13.—Schematic illustration of the diversity of planetary systems
[Kokubo Ida, 2002]
against the initial disk mass for ! 2. The left large circles stand for central
stars. The double circles (cores with envelopes) are Jovian planets, and the
others are terrestrial and Uranian planets. [See the electronic edition of the
Journal for a color version of this figure.]
円盤の質量の違い → ガス惑星の数と位置の違い
22. stirred by interactions between bodies, and
clearing continues through scattering. After
200 million years the inner disk is composed
惑星の移動に伴う惑星系の変化
the collection of planetesimals at 0.06 AU, a
M] planet at 0.12 AU, the hot Jupiter at 0.21
AU, and a 3 M] planet at 0.91 AU. Previous
results have shown that these planets are likely
be stable for billion-year time scales (15).
Many bodies remain in the outer disk, and ac-cretion
and ejection are ongoing due to long
orbital time scales and high inclinations.
Two of the four simulations from Fig. 2
contain a 90.3 M] planet on a low-eccentricity
orbit in the habitable zone, where the temper-ature
is adequate for water to exist as liquid on
a planet_s surface (23). We adopt 0.3 M] as a
lower limit for habitability, including long-term
climate stabilization via plate tectonics (24).
The surviving planets can be broken down into
three categories: (i) hot Earth analogs interior to
the giant planet; (ii) Bnormal[ terrestrial planets
between the giant planet and 2.5 AU; and (iii)
outer planets beyond 2.5 AU, whose accretion
has not completed by the end of the simulation.
Properties of simulated planets are segregated
(Table 1): hot Earths have very low eccentric-ities
and inclinations and high masses because
Fig. 1. Snapshots in time of the evolution of one simulation. Each panel
plots the orbital eccentricity versus semimajor axis for each surviving body.
The size of each body is proportional to its physical size (except for the
giant planet, shown in black). The vertical ‘‘error bars’’ represent the sine
of each body’s inclination on the y-axis scale. The color of each dot
corresponds to its water content (as per the color bar), and the dark inner
dot represents the relative size of its iron core. For scale, the Earth’s water
content is roughly 10j3 (28).
8 SEPTEMBER 2006 VOL 313 SCIENCE www.sciencemag.org
タイプ I, II 惑星落下に
より惑星系の軌道が大き
くかき乱される
多様な惑星系形成
they accrete on the migration time scale (105
years), so there is a large amount of damping
during their formation. These planets are remi-niscent
of the recently discovered, close-in 7.5 M]
planet around GJ 876 (25), whose formation is
also attributed to migrating resonances (26).
[Raymond et al., 2006]