This week’s image is not a true
colour photograph. In fact it isn’t a photograph at all, but rather the results
of a model of atmospheric temperature. The planet being modelled is further
away than any of the objects I’ve talked about before on this blog. This is the
snappily named HD 80606b a “hot Jupiter” gas giant planet in orbit around the star
Struve 1341B. This means that it is similar in size and mass to Jupiter, but
orbits much closer to its sun. If it were in our solar system it would be
inside the orbit of the Earth. It is located in the constellation of Ursa Major
and is approximately 190 light years from Earth. The model shown above was
created using data from NASA’s Spitzer Space Telescope. Exoplanets, those which
orbit other suns, are too far away for us to clearly image them as we would
with a planet in our own solar system. Nonetheless it is amazing how much
information we can gather using a variety of astronomical techniques.
For a long time it was uncertain
whether there actually were planets orbiting other stars. We presumed that our
solar system was the norm, but had very little evidence to back this up. This all
changed in the mid 1990’s when astronomers confirmed the presence of extrasolar
planets, since then we have detected, and confirmed the existence of more than
3000 exoplanets of varying sorts. These include small objects, around twice the
mass of Earth’s Moon, to massive objects which dwarf Jupiter. HD 80606b is four
times as massive as Jupiter, although its radius is slightly smaller. This
means that although it is also a gas giant, it is much denser.
So how do we tell whether planets
orbit a distant star? There are numerous techniques for detecting exoplanets, HD
80606b was detected using the transit method. All planets orbit their host
stars, and this means that if the solar system is angled correctly they will
occasionally pass between their star and Earth. By observing the brightness of
the star over long periods of time, we can detect periodic dimming events,
caused by the planet briefly blocking out some of the light. The extent to
which the light from the star is dimmed can tell us a lot about the physical
properties of the planet, and from the period at which dimming events occur we
can determine the length of time it takes for a planet to orbit its star. Of course
a planet that is far from its star won’t transit very often, and so it might
not be possible to determine the period of the orbit using this method if it
will be hundreds of years before the next transit occurs! As some of the light
from the star passes through the planet’s atmosphere during a transit it can
also give us information about that.
Luckily there are other methods
which can tell us a lot about an exoplanet. For example we can measure changes
in the radial velocity of a star in response to an orbiting planet. All objects
in space respond to the gravity of the bodies around them. A star tends to be
much more massive than its orbiting planets, so the effect which they have on
it is tiny compared to the effect it has on them. Nonetheless it is often
detectable, although it takes very careful measurements to do so. The Earth
only causes a 12 cm/s difference in the speed at which the sun moves relative to
the centre of mass of the solar system, this is tiny, but can be measured. This
means that Earth sized objects can potentially be detected.
HD 80606b has another interesting
property, which brings us back to the model results in this week’s image. It has
one of the most eccentric orbits of any known planet. According to NASA “The
planet spends most of its time far away from its star, but every 111 days, it
swings extremely close to the star, experiencing a massive burst of heat.” This
leads to massive variations in temperature during the course of the orbit, and
this is what is being modelled in the image above. If you follow this link,
they discuss hot Jupiter planets in more detail, and have a video showing the
changes in temperature across the planet as it moves through its orbit. The
image I shared above comes from just after the closest approach, but the planet
soon cools down as it travels further from the star.
Apologies for the delay in posting this week's blog. There will not be a guess the planet in the first week of September, as I'm still rushing around madly this weekend. Things should get back to normal from next Monday, or at least I hope they will!
Image Credit:
NASA/JPL-Caltech/MIT/Principia College
https://www.nasa.gov/image-feature/jpl/simulated-atmosphere-of-a-hot-gas-giant
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