The mass of the initial rocky core determines whether the final planet is potentially habitable. On the top row of the diagram, the core has a mass of more than 1.5 times that of the Earth. The result is that it holds on to a thick atmosphere of hydrogen (H), deuterium (H2) and helium (He). The lower row shows the evolution of a smaller mass core, between 0.5 and 1.5 times the mass of the Earth. It holds on to far less of the lighter gases, making it much more likely to develop an atmosphere suitable for life. Credit: NASA / H. Lammer. Click here for a full resolution image
In the last 20 years the search for
Earth-like planets around other stars has accelerated, with the launch
of missions like the Kepler space telescope. Using these and
observatories on the ground, astronomers have found numerous worlds that
at first sight have similarities with the Earth. A few of these are
even in the ‘habitable zone’ where the temperature is just right for
water to be in liquid form and so are prime targets in the search for
life elsewhere in the universe.
Now a team of scientists have looked at how these worlds form and
suggest that many of them may be a lot less clement than was though.
They find that planets that form from less massive cores can become
benign habitats for life, whereas the larger objects instead end up as
‘mini-Neptunes’ with thick atmospheres and probably stay sterile. The
researchers, led by Dr. Helmut Lammer of the Space Research Institute
(IWF) of the Austrian Academy of Sciences, publish their results in
Monthly Notices of the Royal Astronomical Society.
Planetary systems, including our own Solar system, are thought to
form from hydrogen, helium and heavier elements that orbit their parent
stars in a so-called protoplanetary disk. Dust and rocky material is
thought to clump together over time, eventually forming rocky cores that
go on to be planets. The gravity of these cores attracts hydrogen from
the disk around them, some of which is stripped away by the ultraviolet
light of the young star they orbit.
Dr. Lammer and his team modelled the balance of capture and removal
of hydrogen for planetary cores between 0.1 and 5 times the mass of the
Earth, located in the habitable zone of a Sun-like star. In their model,
they found that protoplanets with the same density of the Earth, but
less than 0.5 times its mass will not capture much gas from the disk.
Depending on the disk and assuming that the young star is much
brighter in ultraviolet light than the Sun is today, planetary cores
with a similar mass to the Earth can capture but also lose their
enveloping hydrogen. But the highest mass cores, similar to the ‘super
Earths’ found around many stars, hold on to almost all of their
hydrogen. These planets end up as ‘mini Neptunes’ with far thicker
atmospheres than our home planet.
The results suggest that for some of the recently discovered
super-Earths, such as Kepler-62e and -62f, being in the habitable zone
is not enough to make them habitats.
Dr. Lammer comments “Our results suggest that worlds like these two
super-Earths may have captured the equivalent of between 100 and 1000
times the hydrogen in the Earth’s oceans, but may only lose a few
percent of it over their lifetime. With such thick atmospheres, the
pressure on the surfaces will be huge, making it almost impossible for
life to exist.”
The ongoing discovery of low (average) density super-Earths supports
the results of the study. Scientists will need to look even harder to
find places where life could be found, setting a challenge for
astronomers using the giant telescopes that will come into use in the
next decade.
The study was carried out by researchers within the Austrian FWF Research Network “Pathways to Habitability”.
Media contact
Dr Robert Massey
Royal Astronomical Society
Tel: +44 (0)20 7734 3307 x214
Mob: +44 (0)794 124 8035
rm@ras.org.uk
Science contact
Dr Helmut Lammer
Mob: +43 316 4120 641
helmut.lammer@oeaw.ac.at
Further information
Notes for editors
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promotes the study of astronomy, solar-system science, geophysics and
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meetings, publishes international research and review journals,
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Source: Royal Astronomical Society (RAS)
