Sky & Telescope - August 2020 - 11

EXOPLANETS

Fomalhaut b: Enshrouded
Exoplanet or Puff of Dust?

FO M A LH AU T: N ASA / ESA / P. K A L AS & J. G R A H A M
(UNIV ERSIT Y OF CA LIFOR NIA , BERK ELE Y ) A ND M. CL A MPIN
(N ASA / GSFC); M A P OF THE M OON: N ASA / GSFC / USGS

DEBATE OVER THE NATURE of Fom-

alhaut b ensued not long after its 2008
discovery. Now, armed with previously
unpublished observations and computer
simulations, András Gáspár and George
Rieke (both at University of Arizona)
make the case in the May 5th Proceedings of the National Academy of Sciences
that Fomalhaut b instead marks the
fast-disappearing remains of a giant
asteroid collision.
The discovery images that Paul Kalas
(University of California, Berkeley) and
colleagues took revealed a spot of visible
light moving about 115 astronomical
units (a.u.) from its star. But the spot
didn't appear at infrared wavelengths,
suggesting its mass was small, perhaps
too small to be a planet at all.
Gáspár and Rieke analyzed previously unpublished Hubble images
obtained in 2013 and 2014, ﬁnding that
Fomalhaut b appears to be moving out
of the system, elongating slightly as it
does so. They also ﬁnd that the object
had faded out of view by 2014.
Gáspár and Rieke propose that
the spot marks a spectacular crash
between two large asteroids, both bigger than 200 kilometers (125 miles)
across. This scenario would explain the
object's trajectory better than if it were
a dust-enshrouded planet, though the
difference between the two scenarios'
predictions is small.
However, such collisions are rare
even in systems as young as 440 million-year-old Fomalhaut. "The planetesimal collision that could create a Fomalhaut b dust cloud would happen once
every 100,000 years, and the dust cloud
can only be seen for 10 years," Kalas
points out. Indeed, Gáspár and Rieke
calculate the collision happened just 39
days before the ﬁrst observations. "Was
I really the luckiest astronomer in the
world when I pointed the Hubble Space
Telescope at Fomalhaut?" Kalas asks.
"Sometimes you do get lucky and
observe a rare event," Gáspár counters.
And some factors, such as migrat-

S The position of Fomalhaut b is marked in this Hubble image from 2008, where the star itself is
blacked out. A thin oval dust ring is also clearly visible around the star.

ing giant planets in the system, could
make massive collisions more common.
Scott Kenyon (Center for Astrophysics,
Harvard & Smithsonian), who was not
involved in the study, says the scenario
is plausible.

Kalas and Gáspár are planning to
follow-up with Hubble and the James
Webb Space Telescope, respectively. "I
certainly wouldn't mind our model
being proven wrong," Gáspár says.
■ MONICA YOUNG

New Geologic Map of the Moon
The U.S. Geological Survey (USGS) has released a new geologic map of
the Moon (1:5,000,000 scale). To code geologic areas, USGS scientists
used information from six Apollo-era regional maps along with topographical data from Japan's Selene mission and NASA's Lunar Reconnaissance Orbiter. Historical maps were aligned with modern data sets
to preserve previous observations. In addition to merging new and old
data, USGS researchers also developed a uniﬁed description of the
stratigraphy, or rock layers, of the Moon, depicted as 48 colors on the
map. This image only hints at the map's riches. To see the map in full
detail, and with a full legend showing what each color indicates, visit the
agency's website at https://is.gd/USGSMoonMap.
■ MONICA YOUNG

sk yandtelescope.org * AUGUST 2 02 0

https://www.is.gd/USGSMoonMap http://www.skyandtelescope.org

Sky & Telescope - August 2020

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