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Changing phases of CoRoT-1b April 13, 2009

Posted by CosmicThespian in Discoveries, Planet Gallery, Space Missions.

400 years ago, Galileo became the first person to observe the phases of Venus.  These observations were proof that the Sun was at the center of the Solar System, not the Earth, and a revolution in our understanding of the cosmos was begun.  Today we are able to observe the changing phases of planets orbiting distant stars!  In a recent paper, astronomers at the Leiden Observatory in the Netherlands published evidence of the different phases of an exoplanet called CoRoT-1b.

CoRoT-1b is the first planet discovered by the CoRoT mission.  CoRoT is a space telescope mission led by the French Space Agency (CNES) which will, among other things, search for exoplanetary transits – the brief dimming of a star’s light as the planet passes between us and its host star.  The planet was discovered in 2007 orbiting a star, CoRoT-1, that is very similar to our Sun.  Planet and star are located 1500 light years away in the constellation Monoceros which means the light we are now seeing left the system roughly 30 years after the fall of the Roman Empire!

While the star may look familiar to humans, the planet is unlike anything in our Solar System.  Almost the same mass as Jupiter and 50% wider, this distant gas planet orbits just 2 million miles from its sun which is only 2% of the distance between Earth and our Sun!  On such a tight orbit, the planet whips around its star in just one and a half days!!  For comparison, Mercury orbits 36 million miles from the Sun once every 88 days.

Sitting so close to its sun has set CoRoT-1b to a slow broil.  Daytime temperatures hover at just under 4000 degrees Farenheit! The intense heat from CoRoT-1 has caused its atmosphere to puff up giving it its unusually large size.


This figure, taken from the linked paper, shows what the authors believe is evidence of the changing phases of this bloated Jupiter as it swings around CoRoT-1.  The top panel is what astronomers call the star’s “light curve” – a plot of the star’s light as it changes over time.  Every dot is a different measurement of the star’s brightness.  The two big dips in the curve at the edges occur when the planet passes in front of the star and blocks a small fraction (about 2%) of its light.

The middle panel shows the same thing but “zoomed in” so that you can see more subtle changes in the star’s light.  They’ve also taken the observations and “binned” them; that is, they break up the observations into evenly spaced chunks (or bins) of time and then take the average brightness within each chunk.  What you see is that, between the transits, the combined light of the star and planet gradually increases by a whopping one-hundredth of a percent.  At a time exactly midway between the transits, there is a sharp decrease in the brightness back to the normal level of the star and then the brightness jumps up again and slowly decreases by the same amount until the next transit occurs.

That looks pretty wierd!

Until you look at the picture at the bottom!  The strange shape of the star’s light curve is consistent with the changing phase of CoRoT-1b as it orbits its sun!  During the transit, the planet has its back to us.  We are looking at its night time and the planet is blocking light from the star.  As soon it moves out of the way, the star light jumps back up.  As the planet swings around on its orbit, we see more and more of its day side (in the exact same way that we see the phase of the Moon or Venus change!).  Since the day side is brighter, the more of it we see, the more light we receive.  Just as we’re about to see the full face of the planet, it disappears behind the star.  The star has eclipsed the planet, an event astronomers usually call a secondary transit.  When that happens, we’re only seeing the light from the star.  When the planet reappears on the other side of the star, we suddenly get light from both star and planet and the light jumps up again.  As the planet continues back around, we see less and less of the day side and thus the combined light from planet and star decreases until the planet once again transits its star and the light plummets.

Observations like this are important tests of our planet detections.  Both doppler measurements and transits can be tricked by normal activity on the surface of the star.  These data provide additional confirmation that we are really seeing the light from a world 9 quadrillion miles away!

I think Galileo would be very proud.

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