A retrograde hot Jupiter: the transiting giant planet orbits very close to the star and in a direction opposite to the stellar rotation. This peculiar configuration results from gravitational perturbations by another much more distant planet (upper left). Credit: Lynette Cook |
More
than 500 extrasolar planets–planets that orbit stars other than the
sun–have been discovered since 1995. But only in the last few years
have astronomers observed that in some of these systems, the star is
spinning one way and the planet is orbiting that star in the opposite
direction.
“That’s
really weird, and it’s even weirder because the planet is so close to
the star,” said Frederic A. Rasio, a theoretical astrophysicist at
Northwestern University. “How can one be spinning one way and the other
orbiting exactly the other way? It’s crazy. It so obviously violates our
most basic picture of planet and star formation.”
The
planets in question are typically huge planets called “hot Jupiters”
that orbit in very close proximity to their central star. Figuring out
how these huge planets got so close to their stars led Rasio and his
research team to also explain their flipped orbits. Details of their
discovery are published in the May 12th issue of the journal Nature.
“And
this discovery is a broader impact of NSF’s MRI program support for the
acquisition of a computer cluster” said Beverly Berger, an NSF
Gravitational Physics Program director. Using it, and performing
large-scale computer simulations, Rasio researchers became the first to
model how a hot Jupiter’s orbit can flip and go in the direction
opposite to the star’s spin. Gravitational perturbations by a much more
distant planet result in the hot Jupiter having both a “wrong way” and a
very close orbit.
“Once
you get more than one planet, the planets perturb each other
gravitationally,” Rasio said. “This becomes interesting because that
means whatever orbit they were formed on isn’t necessarily the orbit
they will stay on forever. These mutual perturbations can change the
orbits, as we see in these extrasolar systems.”
In
explaining the peculiar configuration of an extrasolar system, the
researchers also have added to our general understanding of planetary
system formation and evolution and reflected on what their findings mean
for the solar system.
“We
had thought our solar system was typical in the universe, but from day
one everything has looked weird in the extrasolar planetary systems,”
Rasio said. “That makes us the oddball really. Learning about these
other systems provides a context for how special our system is. We
certainly seem to live in a special place.”
The
physics the research team used to solve the problem is basically
orbital mechanics, Rasio said, the same kind of physics NASA uses to
send satellites around the solar system.
“It
was a beautiful problem,” said Naoz, “because the answer was there for
us for so long. It’s the same physics, but no one noticed it could
explain hot Jupiters and flipped orbits.”
“Doing
the calculations was not obvious or easy,” Rasio said, “Some of the
approximations used by others in the past were really not quite right.
We were doing it right for the first time in 50 years, thanks in large
part to the persistence of Smadar.”
“It
takes a smart, young person who first can do the calculations on paper
and develop a full mathematical model and then turn it into a computer
program that solves the equations,” Rasio added. “This is the only way
we can produce real numbers to compare to the actual measurements taken
by astronomers.”
In
their model, the researchers assume a star similar to the sun, and a
system with two planets. The inner planet is a gas giant similar to
Jupiter, and initially it is far from the star, where Jupiter-type
planets are thought to form. The outer planet is also fairly large and
is farther from the star than the first planet. It interacts with the
inner planet, perturbing it and shaking up the system.
The
effects on the inner planet are weak but build up over a very long
period of time, resulting in two significant changes in the system: the
inner gas giant orbits very close to the star and its orbit is in the
opposite direction of the central star’s spin. The changes occur,
according to the model, because the two orbits are exchanging angular
momentum, and the inner one loses energy via strong tides.
The
gravitational coupling between the two planets causes the inner planet
to go into an eccentric, needle-shaped orbit. It has to lose a lot of
angular momentum, which it does by dumping it onto the outer planet. The
inner planet’s orbit gradually shrinks because energy is dissipated
through tides, pulling in close to the star and producing a hot Jupiter.
In the process, the orbit of the planet can flip.
Only
about a quarter of astronomers’ observations of these hot Jupiter
systems show flipped orbits. The Northwestern model needs to be able to
produce both flipped and non-flipped orbits, and it does, Rasio said.
The
title of the paper is “Hot Jupiters From Secular Planet-Planet
Interactions.” In addition to Rasio and Naoz, other authors of the paper
are Will M. Farr, a postdoctoral fellow at the Center for
Interdisciplinary Exploration and Research in Astrophysics (CIERA);
Yoram Lithwick, an assistant professor of physics and astronomy; and
Jean Teyssandier, a visiting pre-doctoral fellow, all from Northwestern.
The
National Science Foundation, Northwestern’s CIERA and the Peter and
Patricia Gruber Foundation Fellowship supported the research.