An Earth-like planet is likely forming 424 light-years
away in a star system called HD 113766,
say astronomers using NASA's Spitzer Space Telescope.
Scientists have discovered a huge belt of warm dust
— enough to build a Mars-size planet or
larger — swirling around a distant star that is just
slightly more massive than our sun. The dust belt,
which they suspect is clumping together into planets, is
located in the middle of the system's
terrestrial habitable zone. This is the region around a
star where liquid water could exist on any rocky
planets that might form. Earth is located in the middle of
our sun's terrestrial habitable zone.
At approximately 10 million years old, the star is
also at just the right age for forming rocky
planets.
"The timing for this system to be building an Earth is
very good," said Carey Lisse, of Johns
Hopkins' Applied Physics
Laboratory. "If the system was too young, its
planet-forming disk would be
full of gas, and it would be making gas-giant planets like
Jupiter instead. If the system was too old,
then dust aggregation or clumping would have already
occurred and all the system's rocky planets
would have already formed."
According to Lisse, the conditions for forming an
Earth-like planet are more than just being in
the right place at the right time and around the right
star; it's also about the right mix of dusty
materials.
Using Spitzer's infrared spectrometer instrument, he
determined that the material in HD
113766 is more processed than the snowball-like stuff that
makes up infant solar systems and comets,
which are considered cosmic "refrigerators" because they
contain pristine ingredients from the early
solar system. However, it is not as processed as the stuff
found in mature planets and the largest
asteroids. This means the dust belt must be in a
transitional phase, when rocky planets are just
beginning to form.
How do scientists know the material is more processed
than that of comets? From missions like
NASA's Deep Impact — in which an 820-pound impactor
spacecraft collided with comet Tempel 1 —
scientists know that early star systems contain a lot of
fragile organic material. That material includes
polycyclic aromatic hydrocarbons (carbon-based molecules
found on Earth on charred barbeque grills
and in automobile exhaust), water ice and carbonates
(chalk). Lisse says that HD 113766 does not
contain any water ice, carbonates or fragile organic
materials.
From meteorite studies on Earth, scientists also have
a good idea of what makes up asteroids,
the more processed rocky leftovers of planet formation.
These studies tell us that metals began
separating from rocks in Earth's early days, when the
planet's body was completely molten. During this
time, almost all the heavy metals fell to Earth's center in
a process called "differentiation." Lisse says
that, unlike planets and asteroids, the metals in HD 113766
have not totally separated from the rocky
material, suggesting that rocky planets have not yet
formed.
"The material mix in this belt is most reminiscent of
the stuff found in lava flows on Earth. I
thought of Mauna Kea material when I first saw the dust
composition in this system; it contains raw
rock and is abundant in iron sulfides, which are similar to
fool's gold," said Lisse, referring to a well-
known Hawaiian volcano.
"It is fantastic to think we are able to detect the
process of terrestrial planet formation," he
said. "Stay tuned; I expect lots more fireworks as the
planet in HD 113766 grows."
Lisse has written a paper on his research that will be
published in an upcoming issue of
Astrophysical Journal, and he will present his findings
next week at the American Astronomical
Society Division for Planetary Sciences meeting in Orlando,
Fla. Lisse's research was funded through a
Johns Hopkins Applied Physics Laboratory Stuart S. Janney
Fellowship and a Spitzer Space Telescope
guest observer grant.