I am a theoretical astrophysicist and my current
interests focus on planets: both planets in our solar system and ones
that orbit other stars. I study: interior of Jovian planets, planets
in binary systems, circumstellar debris disks (solid and gaseous
components), Pluto-Charon system, primordial Kuiper belt. The
following is a (somewhat dated) popular description of some of my
interests.
Background
While
we have known for a long time that the solar system have 9 planets
(Pluto, the last one discovered at 1930, however, has recently
been demoted from the planetary rank), the search for
planets around other stars has failed until 1995. But since then, we
have been discovering almost 20 'solar systems' each year.
It
is very likely that we are seeing just the 'tip
of the iceberg' -- the planets we found today are
mostly massive planets like Jupiter, Earth-like planets are too puny
and are yet to be detected. Moreover, these systems are not what we
expected based on our local experience. While Jupiter is orbiting the
Sun at a safe distance of 5 astronomical unit (1 AU is the
distance between Earth and Sun), many of these planets
are so close to their host stars (down to 0.03 AU)
they risk being swallowed or being evaporated. Most of them also have
highly eccentric orbits -- one planet in particular has such a high
eccentricity that its orbit resembles more that of a comet than that
of the Jupiter.
With the increasingly large sample of
extra-solar planets, we urgently need a unifying theory for planet
formation, one that can explain both the taxonomy of solar system
planets, as well as the exotic behavior in these extra-solar systems.
Such a theory will reliably predict both the frequency of occurrence
as well as the location of earth-like planets around stars.
Planets
in Binary Systems
Many of these planets are also
found in stellar binaries with the two stars separated by as little
as 20 AU, and this raises the question about the role of stellar
binarity in planet formation and evolution. We have good reasons to
wonder about this.
Astronomers now know that stars like the
Sun, which floats alone in the interstellar space is a rarity. At
least 80% stars are in binary systems or triple-star systems. This
number is set to rise once we have better techniques to tell apart
two close-by stars. So it is well likely that most planets in the
galaxy are not formed around sedate single stars, but around swirling
double (or triple) systems. If this conjecture is correct, it will
naturally explain the differences between the solar system and the
majority of extra-solar systems.
Together with Norm Murray
(CITA) and various students, we have investigated the effect of
binarity on planet migration. A planet formed at Jupiter's location
from a star (~5 Astronomical Units), under the gravitational
influence of a companion star which is orbiting at a different plane
as the planet does, may undergo drastic inward migration. We call
this the Kozai migration. This mechanism may help explain the
existence of hot Jupiters. The planet's orbit may also be excited to
high values.
Moreover, I would like to pursue a series of
related questions: how does a binary companion affect the dynamics
inside a protoplanetary disk? will it tend to accelerate or inhibit
planet formation in the disk? what causes the relative orientation
between the binary plane and the disk plane, etc. I will use mostly
semi-analytical methods, with some support of computer simulations. I
am also collaborating with observational astronomers trying to
uncover some basic facts in binary stars. We are starting the largest
ever binary survey to disclose the underlying distribution of stellar
binaries, aiming to answer questions like, do two stars in a binary
tend to have the same mass, what is the average separation of binary
stars, do lighter stars couple as much as massive stars, etc.
Tidal
Dissipation in Jupiters
The second component of my
research program focuses on the tidal dissipation process, arises
from a 40-year old puzzle: the tide raised by Jupiter's satellite
(Io) on Jupiter appears to be dissipated much more efficiently than
expected.
In the Earth-Moon system, the Moon is receding away
from us at a rate of ~ 3.8 cm/year. This is caused by the dissipation
of lunar tide in the rotating Earth -- the Moon spins down the Earth
and at the same time it gets thrown out. The same process is
happening between Jupiter and its natural satellites (especially with
Io). From this we infer the rate of tidal dissipation in Jupiter.
However, the actual dissipation is some 106 times more
efficient than any reasonable theories we have come up with. This
discrepancy may imply that our understanding of Jupiter's interior is
incomplete, or some radically new physical process is required.
It
is very interesting to notice that the same discrepancy exists in
extra-solar planets. From their orbital characteristics, we infer
that their tidal dissipation efficiency is similar to that of
Jupiter. The surface of these planets are heated by their host stars
to ~ 1200 Kelvin, while the surface temperature of Jupiter is 120
Kelvin. These planets may not have exactly the same chemical
composition, solid core sizes, etc., as that of Jupiter. So whatever
contributes to the tidal dissipation must be something that is rather
basic and depends only on the rough structure of the planets.
Another reason why I am interested in this issue is that the
same old discrepancy again shows up in sun-like stars. Solar-type
stars are observed to dissipate tides (and therefore circularize
their orbits) at a rate comparable to that of giant planets. Many
excellent theorists have made unsuccessful attempts at this problem.
Finally solving it will be an exciting intellectual success.
With
a collaborator, we have made some break-through progress in this area
and believe we have finally pinpointed the responsible physical
mechanism. This is related to the special behavior of fluid when it
is rotating. A new branch of internal waves exist in such a fluid
that can become resonant with the tidal perturbations. Just like
resonant sloshing of water in a tub can give rise to large
amplitudes, the resonance helps to absorb more energy out of the
tidal potential and then this energy is dissipated very near the
surface of the fluid. The new branch of mode and the resonance
explain the much larger dissipation efficiency than expected (from a
non-rotating fluid). We are in the process of elucidating this
mechanism, as well as applying it to both Jupiter, exo-jupiters, and
stars.
The intellectual benefit of such a theory does not
stop at explaining things. It is a probe deep into the interior of
these bodies, like few other processes can. In this way, we may hope
to discern the inner make-up of extra-solar planets, and provide fuel
for distinguishing different planet formation theories.
Pluto-Charon binary
Circumstellar Debris Disks
Summary
My
over-all goal to understand how planets come about, this includes
studying how environmental effects come into play, what is the
interior structure of planets, etc. This is currently a very exciting
and competitive area and new discoveries are made daily. Research in
this area helps to shape our view on how we ourselves come about.