Download Could there be life on exoplanets? No room for complacency

VIEWS
George Cole
Could there be life on exoplanets?
Two very interesting articles in the
December issue, “The astrochemistry of life” (Richards and Sarre
A&G 42 6.17) and “Solar evolution
and the distant future of the Earth”
(Schröder, Smith and Apps A&G 42
6.26), together raise several important questions. The discovery of
planetary companions of some
solar-type stars suggests that some
of these so-called exoplanets may
carry living materials and, especially, technologically capable life. Can
any indications be inferred from the
present knowledge of these systems?
Living materials as we know them
require liquid water and a constant,
moderate source of heat. For the
Sun, this restricts the habitable zone
for advanced life to an orbital band
within about 0.7 and 1.5 AU. The
major planets beyond act as a gravitational “shield”. The same conditions apply to other stars. They are
all associated with a finite lifetime.
Whether microbial life formed on
Earth or whether precursors came
from outside (transpermia), elemen-
tary life was certainly present very
soon after the formation of the
Earth. Advanced and technologically-capable creatures required a
much longer time to appear. Long
timescales on Earth imply a lengthy,
stable environment of solar radiation, stable planetary orientations
and gravitational protection of the
planet by one or more larger planets in exterior orbits. By implication, the orbit of the larger planet
must have a semi-major axis at
least as large as 2 AU and preferably rather larger. With the solar
system in mind, interest now is with
companions of this semi-major axis
and of mass broadly comparable to
that of Jupiter, MJ = 1.89 ×1027 kg.
Observations so far suggest that
the exoplanet systems have three
general characteristics. First, there
is usually one observed companion.
Second, the semi-major axes are
generally small, less than 0.1 AU.
This leaves little room for smaller
planets inside. Third, the eccentricities of the orbits are often not small
but have values similar to those
found for comets in the solar system. These characteristics seem
incompatible with the conditions
necessary for the development of
advanced and technological life.
There are, however, six systems
where this is not necessarily the
case (listed in table 1). While they
each have a planet with a semimajor axis >2 AU, only one has a
small eccentricity. 47 Ursa Major
has two large companions with
semi-major axes >2 AU, each with
an essentially circular orbit. This
star is 14 pc from Earth, which is
about 44 ly (1 ly = 6.324 ×104 AU).
The intense search for exoplanets
since 1995 should have revealed
companions with semi-major axes
at least as large as 6 AU, but none
have been found so far. It might be
concluded that companions with
physical characteristics able to support advanced life are few. Might
there be a sphere of radius about
44 ly around the Earth where
advanced life forms are very unlikely? This is a cell of volume
3 ×105 (ly)3 which might contain
one “civilization”. The mean volume of an average galaxy is of
Table 1: Star/companion systems with large semi-major axes
System
Stellar distance (pc)
ε Eridani
47 Ursa Major
14 Her
HD 190228
HD 10697
υ Andromeda
MJ = 1.89 ×1027 kg
Min. planet mass (MJ)
3.0
14.08
0.76
18.15
62.11
30.0
13.47
2.11
4.61
1 AU = 1.496 ×108 km
Semi-major axis (AU)
0.8
2.54
3.73
3.3
4.99
6.59
0.71
0.83
2.5
1 pc = 2.06 ×105 AU
3.4
2.09
< 0.2
2.5
2.31
2.0
0.059
0.18
0.41
Amanda Baker and David Clements
No room for complacency
We were delighted to see several letters in the December 2001 issue of
A&G, responding to our summary
of the NAM2001 Careers Session.
It was particularly gratifying to
receive such enthusiastic support
from three such diverse parties;
indeed we, and I’m sure several
others, were most tempted by Dr
Harper’s description of life in
bio-informatics.
However, we are extremely disappointed (although, perhaps, not so
very surprised) by the glaring
absence of any response from
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PPARC, any heads of department,
or anyone else who sets policy concerning graduate students, postdocs
and other fixed-term contract
researchers (FTCS). This could be
seen as an indication that policy
makers have no interest in engaging
with, let alone tackling, the very
serious issues which we, and the
other correspondents, raised.
Would it be unfair to conclude
that established academics have a
deeply vested interest in maintaining the current iniquitous and damaging status quo? After all, those
Orbital eccentricity
0.6
0.061
0.35
0.43
0.12
0.034
order 1015 (ly)3. There would be of
the order 3 ×109 (1015/ 3 ×105) such
cells in the galaxy. How many
might contain a civilization? The
answer would give an indication of
the number of life forms present,
independent of the Drake equation.
Fossil remains of life on Earth
reveal that microbial life appeared
very soon after the formation of the
planet, 4500 million years ago, and
seems almost indestructible.
Advanced life, by contrast, has been
affected strongly by changing geophysical conditions. There have
been several major extinctions (easily survived by microbia), and homo
sapiens has appeared after a series of
unlikely events – apparent accidents.
It is possible that evolutionary forces
and external physical events will
limit the life span of the species. The
Sun itself has only a finite life anyway. Such considerations can be
expected to apply elsewhere, suggesting that microbial life might be
universal but advanced life, being
vulnerable, may be rare and limited.
If one cell in a hundred develops
advanced animate forms, there
could be of the order 3 ×107 “civilizations” in the galaxy. If only one
in a million does so, the number is
approximately 3 ×104. Each has a
finite lifetime, though it may be
lengthy. These are relatively small
numbers; they become enormously
large if other galaxies are included.
The mean distance between cells
remains always large, effectively
isolating them. There would, anyway, be no evolutionary advantage
of contact-at-a-distance since there
is then no mixing of genes, necessary for a universal application of
Darwinian arguments.
G H A Cole, University of Hull, Hull,
HU6 7RX. [email protected]
privileged few academics on openended contracts can thereby acquire
cheap, virtually disposable labour
to work on their research projects,
and advance their RAE ratings,
with no need to devote significant
effort to the career development or
long-term aspirations of at least
three-quarters of their graduate students and FTCS.
While the infamous Concordat is
meant to address some of these
issues, it is telling to note that a
recent study (Academic Research
Careers in Scotland, SHEFC)
showed that the majority of contract researchers do not even know
of its existence. The reality on the
ground is that academics routinely
fail to provide adequate (or indeed,
any) career guidance for their graduate students and FTCS; meanwhile, university careers services are
woefully ill-prepared to meet the
very real needs of these people
when their research supervisors fail
them. In fact, only 12% of contract
researchers in the SHEFC study
received any form of formal careers
advice or guidance.
If there is a real desire for reform
on the part of the astronomy and
geophysics “establishment”, much
more must be done and there must
be a dialogue between those who
set policy and those who are subject
to fixed-term contracts. This will be
especially true as universities and
February 2002 Vol 43
VIEWS
M Iftikhar Ahmad
The New World Calendar
The Gregorian calendar was introduced in March 1582 by Pope
Gregory XIII. It was subsequently
adopted by various countries at
different times. Great Britain
adopted it on 2 September 1752,
whereas Russia didn’t adopt it until
1917. At present almost every
country in the world uses this
calendar.
The Gregorian calendar has a
number of drawbacks. The Earth
revolves around the Sun and
completes one revolution in 365
days, 5 hours, 48 minutes and 46
seconds; i.e. the length of one
tropical year is 365.2422 days. This
fractional part makes the formulation of the calendar difficult –
although the Gregorian calendar is
better than the Julian calendar, as
the average duration of the year
improved from 365.2500 to
365.2425 days. The difference,
therefore, between the average
duration of the Gregorian year and
the length of the tropical year
remains 0.0003 days instead of
0.0078 days. However, this means
that the Gregorian calendar is out
by 0.0003 days per year, which will
amount to 3 days in 10 000 years.
A calendar needs to be devised in
such a way that the average duration of a year is exactly equal to the
length of a tropical year.
A solution of the above problem is
given in “Refinement of the
Gregorian calendar” (M Iftikar
Ahmad 1992 J. Sci. Res. P U Lahore,
Pakistan XXI 1&2 57–63) where
the following formula is suggested:
A year divisible by four will be a
leap year, provided that it is not a
multiple of 100. In the case where a
year is a multiple of 100, it will be a
leap year only if the number
obtained by suppressing the zeros
on the right is divisible by four.
By this formula, the years 2000,
6000 and 10 000 would not be leap
years, despite being divisible by
400, as they are according to the
Gregorian calendar. In this way, the
3 days difference in 10 000 is
adjusted. Similarly, during the
following 10 000 years, the years
14 000, 18 000 and 20 000 would
not be leap years, and the average
duration of the calendar year is
reduced to 365.2422 days per year.
This agrees with the length of a
tropical year.
The formulae set out above are
based on the assumption that the
length of a tropical year is constant,
whereas according to the
Explanatory supplement to the
Astronomical Ephemeris and the
American Ephemeris and Nautical
Almanac, 1961 it is shrinking and
even the new formula given above
will eventually get out of step. The
shrinkage according to the formula
is given by table 1, where T is the
time in Julian centuries from the
epoch; the terms in T2 and T3 in the
1992 edition are too small to affect
the discrepancy. This means that
the tropical year is shortening in
days by 0.0000614 per millennium.
The Gregorian calendar is haphazard in the sense that the number
of days in a month varies from 28
to 31. They should be equal in
length as far as possible. It is
important from the point of view of
business and banking that the year
research councils change their policies to respond to the new regulations from the EU Directive on
Fixed Term Work (www.dti.gov.uk/
er/fixed/) which, at face value,
would suggest that many of the
iniquities arising from fixed-term
contracts are to be made illegal.
These issues go to the heart of
the way science operates in the UK.
The problems produced by inappropriate dependence on fixedterm contracts are now being
examined by several influential
bodies beyond astronomy and
geophysics (e.g. SHEFC report,
Quinquennial Review of Research
Councils, The Roberts’ Review)
and are being raised at the highest
levels (e.g. the Scottish Parliament,
the House of Lords).
Comparison with the USA is
telling. According to preliminary
results of a longitudinal AAS study
(Marvel et al., private communication) 68% of astronomy PhDs are
still working in the subject seven to
nine years after completing their thesis. This compares to PPARC’s figure of 25% of UK astronomy PhDs
obtaining “permanent posts” in the
field. The simple conclusion from
these figures is that anyone wanting
a long-term career in astronomy
should avoid UK universities and
move to the US for their PhD.
And yet we hear nothing from
PPARC or anyone else in the
February 2002 Vol 43
Table 1: Shortening of the tropical year
Edition Epoch Days
1974 1900 365.24219878 – 0.00000614 T
1992 2000 365.2421896698 – 0.00000615359 T – 7.29 ×10–10 T 2 + 0.000…
Table 2: Accounting for the shortening
tropical year
i
1
2
3
4
5
6
7
Pi
AD 2001–7000
AD 7001–12000
AD 12001–17000
AD 17001–22000
AD 22001–27000
AD 27001–32000
AD 32001–37000
λi
3200
2800
2400
2000
1600
1200
800
should be divided into as near as
possible four equal quarters, each
of three months. In the Gregorian
calendar, the first, second, third and
fourth quarters of a normal year
are of 90, 91, 92 and 92 days
respectively.
In view of all these deficiencies,
the Gregorian calendar is not
compatible with the present age,
and in the New World order, a new
calendar is required. This calendar
should be devised in such a way
that it removes the defects of the
present calendar and at the same
time is simple, regular, uniform and
meets all other requirements as far
as possible. Keeping in view all
these considerations, the following
calendar is suggested:
● The first month of every quarter
and the last month of the year, i.e.
January, April, July, October and
December, will each have 31 days.
● The remaining months, i.e.
February, March, May, June,
August, September and November,
will each have 30 days.
● In leap years, June will have 31
days. By increasing the length of
June by one day, the quarters of 91
and 92 days will be placed alternately during the leap year.
To make this calendar conform
with the changing tropical year, the
following formula is suggested:
● A year divisible by four will be a
leap year, provided it is not a
multiple of 100.
● In the case where a year is a
multiple of 100, then it will be a
leap year during the period Pi if it is
divisible by 400 but not by λi ,
where λi forms an arithmetic
progression as given in table 2.
Note that by the year AD 37 000,
the length of a tropical year will
become 365.2400 and no century
year will be a leap year.
astronomy and geophysics “establishment” concerning these issues.
If this silence genuinely reflects the
level of interest that PPARC has in
the career development of its postdocs, then it is probably time for us
all to contact organizations such as
The Sanger Institute and look into
new areas of work in which something deserving of the name
“career” is possible.
Academic Research Careers in Scotland
www.shefc.ac.uk/content/shefc/research/crs/
CRS.html
DTI Website www.dti.gov.uk/er/fixed/
Quinquennial Review of the Research Councils 2001, DTI.
Sir Gareth Roberts Review of the Supply of
Scientists and Engineers (due to report for HM
Government in Feb 2002) www.hm-treasury.gov
.uk/Consultations_and_Legislation/ Review_of_
the_Supply_of_Scientists_and_Engineers/
Motions to Scottish Parliament S1M–2184
and S1M–2368 Hansard 28 11 2001, Col 369.
Amanda Baker, Cardiff; David
Clements, London.
References
A Concordat to provide a framework for the
career management of contract research
staff in universities and colleges
www.dti.gov.uk/ost/concord.htm
M Iftikhar Ahmad, Lahore, Pakistan.
● Note that this year’s NAM will have
a Careers Session as well as other
forums for discussion of these issues.
www.star.bris.ac.uk/nam/index.html
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