Download notes

Nature of Exoplanets
26 October 2016
Exoplanet Census
• as of 1 October 2016, there have been 3,532
exoplanets in 2,649 planetary systems and 595
multiple planetary systems confirmed.
• Kepler space telescope (since 2009) has found
more than two thousand. Kepler has also
detected a few thousand candidate planets, of
which about 11% may be false positives.
Counts of Exoplanets
Jupiter vs Tres-3
Earth-Like Planets
• About 1 in 5 Sun-like stars have an "Earthsized” planet in the habitable zone
• Assuming there are 200 billion stars in the
Milky Way, one can hypothesize that there are
11 billion potentially habitable Earth-sized
planets in the Milky Way
Remember
• The nearest exoplanet is Proxima Centauri b,
located 4.2 light-years (1.3 parsecs) from Earth
and orbiting Proxima Centauri, the closest star to
the Sun.
• The discovery of exoplanets has intensified
interest in the search for extraterrestrial life.
There is special interest in planets that orbit in a
star's habitable zone, where it is possible for
liquid water, a prerequisite for life on Earth, to
exist on the surface. The study of planetary
habitability also considers a wide range of other
factors in determining the suitability of a planet
for hosting life
Formation and evolution
• Planets form within a few tens of millions of years of
their star forming, and there are stars that are forming
today and other stars that are ten billion years old, so
unlike the planets of the Solar System, which can only
be observed as they are today, studying exoplanets
allows the observation of exoplanets at different stages
of evolution.
• When planets form they have hydrogen envelopes that
cool and contract over time and, depending on the
mass of the planet, some or all of the hydrogen is
eventually lost to space.
• This means that even terrestrial planets can start off
with large radii. An example is Kepler-51b which has
only about twice the mass of Earth but is almost the
size of Saturn which is a hundred times the mass of
Earth
Atmospheres
•Atmospheres have been detected around several
exoplanets. The first to be observed was HD 209458 b in
2001.
•KIC 12557548 b is a small rocky planet, very close to its
star, that is evaporating and leaving a trailing tail of cloud
and dust like a comet. The dust could be ash erupting
from volcanos and escaping due to the small planet's low
surface-gravity, or it could be from metals that are
vaporized by the high temperatures of being so close to
the star with the metal vapor then condensing into
dust.[99]
•In June 2015, scientists reported that the atmosphere of
GJ 436 b was evaporating, resulting in a giant cloud
around the planet and, due to radiation from the host
star, a long trailing tail 14×106 km (9×106 mi) long
Exoplanet Atmospheres
• Several planets outside the Solar System have been
observed to have atmospheres. At the present time, all
atmosphere detections are of hot Jupiters or hot Neptunes
that orbit very close to their star and thus have heated and
extended atmospheres.
• Observations of exoplanet atmospheres are of two types.
First, transmission photometry or spectra detect the light
that passes through a planet's atmosphere as it transits in
front of its star.
• Second, the direct emission from a planet atmosphere may
be detected by differencing the star plus planet light
obtained during most of the planet's orbit with the light of
just the star during secondary eclipse (when the exoplanet
is behind its star)
Atmospheric composition
• In 2001, sodium was detected in the atmosphere of HD 209458 b.
• In 2008, water, carbon monoxide, carbon dioxide and methane were
detected in the atmosphere of HD 189733 b.
• In 2013, water was detected in the atmospheres of HD 209458 b, XO-1b,
WASP-12b, WASP-17b, and WASP-19b.
• In July 2014, NASA announced finding very dry atmospheres on three
exoplanets (HD 189733b, HD 209458b, WASP-12b) orbiting Sun-like stars
• In September 2014, NASA reported that HAT-P-11b is the first Neptunesized exoplanet known to have a relatively cloud-free atmosphere and, as
well, the first time molecules of any kind have been found, specifically
water vapor, on such a relatively small exoplanet
• The presence of oxygen may be detectable by ground-based telescopes,
which, if discovered, would suggest the presence of photosynthetic life on
an exoplanet.
Summary
• Thousands of exoplanets, with many Kepler
candidates still to be confirmed
• Hundreds are in multiple star systems
• About half have eccentric orbits, unlike our
Solar System
• About 1 in 5 sunlike stars have an Earth-sized
planet in the Habitable Zone