Download Can we detect asteroid impacts with rocky extrasolar planets?

The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
Search
Subscribe
Enter your email
address below to
be notified when
new articles are
published:
subscribe
our privacy
policy
Giant impacts on exoplanets might be detectable from Earth, providing
another means to study these worlds. (credit: Don Davis/NASA)
Can we detect asteroid impacts
with rocky extrasolar planets?
by Michael Paine
Monday, December 11, 2006
Caution: This article contains speculation, ballpark
estimates, mathematics, and tables!
Sixty-five million years ago a chunk of rock and ice
perhaps 15 kilometers across collided with the Earth and
wiped out many creatures, including the dinosaurs. This
impact, known as the Chicxulub impact, must have
created a spectacular flash. Was it bright enough to be
detected as far away as Sirius? How many impacts like
this are occurring in our region of the Milky Way? Have
we any chance of detecting these impacts? These
questions can be answered, in part, with a little
mathematics, a few facts from astronomy and the
audacity to assume that our solar system is “typical”.
In 1994 astronomer Alan Stern asked a similar question
and evaluated the detection of newly formed planets that
were still glowing red hot from the accretion process. He
1 of 6
10/12/12 1:23 PM
The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
also calculated the detectability of giant impacts with
planets like Neptune. Building on this work, let us
consider the effects of a Chicxulub-size impact with
planets similar to the Earth.
Impact frequency
The Earth suffers a Chicxulub-size impact every 100
million years or so. There are four rocky planets in our
solar system, so we can expect one to be hit every 25
million years. However, half of these impacts will be on
the side facing away from the Earth so a visible impact
occurs, on average, once every 50 million years. During
the first billion years of our solar system, though, the
impact rate was at least 1,000 times as intensive as the
present rate. This means the average rate over the
four-billion-year life of the solar system is some 250
times the current rate, or one impact every 200,000
years.
Number of stars in our region of the solar system
It is about four light-years to the nearest star beyond our
Sun, but that is fairly close compared with most stars. On
average there is about one star for every 500 cubic
light-years of space. This does not seem like much, but
distance raised to the third power quickly produces large
numbers:
Radius of
sphere (lightyears)
10
100
1000
Volume
Average
Number
(cubic light
frequency of
of stars
years)
impact events
4200
8
26,000 years
4.2 million
8,000
26 years
4.2 billion
8 million 1.5 weeks
So if we could detect a Chicxulub-size impact from 1,000
light-years away then it should only take a few weeks of
observations to find one.
The impact fireball
The impact of an asteroid (or comet) into the surface of a
rocky planet typically generates a hemispherical fireball
with a diameter some 14 times the diameter of the
2 of 6
10/12/12 1:23 PM
The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
asteroid. This means the Chicxulub fireball was about
200 kilometers in diameter (and, as it happens, the
Chicxulub crater has a similar diameter.)
When fully expanded, the
This impact, known
as the Chicxulub
temperature of the gases in the
impact, must have
fireball are about twice the surface created a
temperature of our Sun, or about spectacular flash.
Was it bright
12,000 kelvins. It is also
enough to be
detected as far
convenient to compare the
away as Sirius? How
luminosity of the fireball with that many impacts like
this are occurring
of the Sun. This, in turn, can be
in our region of the
used to compare brightness at a
Milky Way?
distance. The relative luminosity,
or ratio of the fireball luminosity to the Sun’s luminosity,
is proportional to the square of the diameter ratio and
the fourth power of the temperature ratio:
Relative luminosity = (Fireball diameter/Sun diameter)2
x (Fireball temperature/Sun temperature)4
The Chicxulub fireball works out as having a relative
luminosity of 3.6x10-7 (that is one divided by about three
million) so the task of detecting the event a long way
from our solar system seems daunting. However, that is
not the end of the bad news. The fireball tends to mask
itself so that not all of the light is emitted. This emissivity
can drop to one percent or less depending on the
characteristics of the impact.
The good news is that Venus has a luminosity that is
about one billionth of that of the Sun, so if we can detect
a Venus-like planet around another star then it is likely
that we could detect a Chicxulub impact, provided the
detection system was fast enough to record a fireball that
lasts about a minute. There might also be other, more
effective ways to detect the complex radiation from these
impacts but this analysis is confined to visible light.
Detection from a distance
The apparent brightness of stars is measured as
“magnitude”. Magnitude is a type of reverse logarithmic
scale. One interval of magnitude corresponds with a
brightness decrease of a factor of 2.5. The faintest
3 of 6
10/12/12 1:23 PM
The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
naked-eye star has a magnitude of about 6.5. Sirius has a
magnitude of –1.4. For the purpose of comparing the
brightness of stars an absolute magnitude is used and is
based on the brightness of the star if it was viewed from
about 32 light-years away (10 “parsecs”).
We now have enough information to take a guess at the
apparent magnitude of a Chicxulub-size impact at
various distances:
Apparent Magnitude of Chicxulub Impact for Various
Emissivities
Observer’s
distance (light
years)
Object
Relative
Absolute
100 1000
10 ly
(emissivity) Luminosity Magnitude
ly
ly
Chicxulub
20.9
19.7 22.2 24.7
3.6x10-7
(100%)
Chicxulub
22.7
21.4 23.9 26.4
7.2x10-8
(20%)
Chicxulub
25.9
24.7 27.2 29.7
3.6x10-9
(1%)
Sun
1
4.8
3.6 6.1 8.6
Venus
1.16x10-9
27.2
25.9 28.4 30.9
Typical Spaceguard telescope systems (looking for
near-Earth asteroids) have a limiting magnitude
(detection threshold) of about 21 for a 100-second
exposure. Therefore a Chicxulub-size impact viewed from
10 light-years away would be barely detectable (20.9),
even at 100% emissivity. It seems that the Spaceguard
system has no chance of detecting an extrasolar impact.
Telescopes are currently being
The detection of
extrasolar impacts
developed that will look at large
would give scientist
areas of the sky for transient
a better idea of
astrophysical events. For example, whether our solar
system was typical.
NASA’s Swift spacecraft recently In particular it
detected a giant stellar flare some would be an
indicator of the
135 light-years away. These new presence of other
rocky planets.
telescopes should have greater
sensitivity than the Spaceguard
telescopes (many of which are hand-me-downs). It is
4 of 6
10/12/12 1:23 PM
The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
therefore possible that one of these new systems will
eventually come across a major extrasolar impact.
So what: why bother with extra-solar impacts?
First of all, astronomers looking for other types of
transient events should be aware that they may,
serendipitously come across an impact event. They
should understand the likely signatures of impact events.
The detection of extrasolar impacts would give scientist a
better idea of whether our solar system was typical. In
particular it would be an indicator of the presence of
other rocky planets. The characteristics of the impact
flash might also reveal the chemistry of the surface of the
planet, just like the Deep Impact mission that collided
with comet Tempel 1 in July 2005.
Finally, the detection of these events would give a better
understanding of the role that impacts play in the
evolution of planetary surfaces and, perhaps, the
evolution of life on those planets.
Bibliography and acknowledgements
Bondi, H, 1970, “Astronomy of the Future”, Quarterly
Journal of the Royal Astronomical Society, Vol. 11, p.443
(astronomy of the day was missing transient events)
Collins G, Melosh H and Marcus R, 2005, “Earth Impact
Effects Program: A Web-based computer program for
calculating the regional environmental consequences of a
meteoroid impact on Earth”, Meteoritics & Planetary
Science Vol.40 No. 6, pp817–840.
Harris A, 1998, “Evaluation of ground-based optical
surveys for near-Earth asteroids”, Planet. Space Sci. Vol.
46 No. 2/3, pp283-290.
Lewis J, 2000, Comet and asteroid impact hazards on a
populated Earth, Academic Press.
Stern A, 1004, “The detectability of extrasolar terrestrial
and giant planets during their luminous final accretion”,
The Astronomical Journal, Vol. 108 No. 6, pp2312-2317.
5 of 6
10/12/12 1:23 PM
The Space Review: Can we detect asteroid impacts with rock...
http://www.thespacereview.com/article/761/1
Thanks to Duncan Steel, Paul Davies, John Lewis, Alan
Stern and Jay Melosh for advice on this topic.
Michael Paine ([email protected]) is a consulting
mechanical engineer based in Sydney. He maintains the
web pages of the Planetary Society Australian
Volunteers and has been lobbying the Australian
government about the Spaceguard issue since funding
was stopped in 1996.
Home
6 of 6
10/12/12 1:23 PM