Planetary systems with habitable Earths?

More than 100 planetary systems have already been discovered around
distant stars. Unfortunately, the limitations of current technology
mean that only giant planets (like Jupiter) have so far been detected,
and smaller, rocky planets similar to Earth remain out of sight.

How many of the known exoplanetary systems might contain habitable
Earth-type planets? Perhaps half of them, according to a team from the
Open University, led by Professor Barrie Jones, who will be describing
their results today at the RAS National Astronomy Meeting in Milton
Keynes.

By using computer modelling of the known exoplanetary systems, the
group has been able to calculate the likelihood of any ‘Earths’
existing in the so-called habitable zone – the range of distances from
each central star where life as we know it could survive. Popularly
known as the “Goldilocks” zone, this region would be neither too hot
for liquid water, nor too cold.

By launching ‘Earths’ (with masses between 0.1 and 10 times that of
our Earth) into a variety of orbits in the habitable zone and
following their progress with the computer model, the small planets
have been found to suffer a variety of fates. In some systems the
proximity of one or more Jupiter-like planets results in gravitational
ejection of the ‘Earth’ from anywhere in the habitable zone. However,
in other cases there are safe havens in parts of the habitable zone,
and in the remainder the entire zone is a safe haven.

Nine of the known exoplanetary systems have been investigated in
detail using this technique, enabling the team to derive the basic
rules that determine the habitability of the remaining ninety or so
systems.

The analysis shows that about half of the known exoplanetary
systems could have an ‘Earth’ which is currently orbiting in at least
part of the habitable zone, and which has been in this zone for at
least one billion years. This period of time has been selected since
it is thought to be the minimum required for life to arise and
establish itself.

Furthermore, the models show that life could develop at some time
in about two thirds of the systems, since the habitable zone moves
outwards as the central star ages and becomes more active.

HABITABLE MOONS

A different aspect of this problem is being studied by PhD student
David Underwood, who is investigating the possibility that Earth-sized
moons orbiting giant planets could support life. A poster setting out
the possibilities will be presented during the RAS National Astronomy
Meeting.

All of the planets discovered so far are of similar mass to
Jupiter, the largest planet in our Solar System. Just as Jupiter has
four planet-sized moons, so giant planets around other stars may also
have extensive satellite systems, possibly with moons similar in size
and mass to Earth.

Life as we know it cannot evolve on a gaseous, giant
planet. However, it could survive on Earth-sized satellites orbiting
such a planet if the giant is located in the habitable zone.

In order to determine which of the gas giants located within
habitable zones could possess a life-friendly moon, the computer
models search for systems where the orbits of Earth-sized satellites
would be stable and confined within the habitable zone for at least
the one billion years needed for life to emerge.

The OU team’s method of determining whether any putative ‘Earths’
or Earth-sized satellites in habitable zones can offer suitable
conditions for life to evolve can be applied rapidly to any planetary
systems that are newly announced. Future searches for ‘Earths’ and
extraterrestrial life should also be assisted by identifying in
advance the systems most likely to house habitable worlds.

The predictions made by the simulations will have a practical value
in years to come when next-generation instruments will be able to
search for the atmospheric signatures of life, such as large amounts
of oxygen, on ‘Earths’ and Earth-sized satellites.

BACKGROUND

There are currently 105 known planetary systems other than our own,
with 120 Jupiter-like planets orbiting them. Two of these systems
contain three known planets, 11 contain two and the remaining 92 each
have one. All but one of these planets has been discovered by their
effect on their parent stars’ motion in the sky, causing them to
wobble regularly. The extent of these wobbles can be determined from
information within the light received from the stars. The remaining
planet was discovered as the result of a slight dimming of starlight
caused by its regular passage across the disk of its parent star.

Future discoveries are likely to contain a higher proportion of
systems that resemble our Solar System, where the giant planets orbit
at a safe distance beyond the habitable zone. The proportion of
systems that could have habitable ‘Earths’ is, therefore, likely to
rise. By the middle of the next decade, space telescopes should be
capable of seeing any ‘Earths’ and investigating them to see if they
are habitable, and, indeed, whether they actually support life.