Tag: Gaia

Tatooine Worlds

Science fiction has become science.  No habitable planets orbiting two suns like the fictional Tatooine have been detected so far, but more than a dozen “circumbinary planets” have been identified and many more are predicted.  Exoplanets orbiting a host star that orbits its own companion star are even more common. (Lucasfilm)

When the the first Star Wars movie came out in 1977, it featured the now-iconic two-sun, “circumbinary” planet Tatooine.  At that time astronomers didn’t really know if such solar systems existed, with more than one sun and at least one planet.

Indeed, the first extra-solar planet wasn’t detected until the early 1990s.  And the first actual circumbinary planet was detected in 2005, and it was a Jupiter-size planet orbiting a system composed of a sun-like star and a brown dwarf.  Tatooine was definitely not a Jupiter-size planet.

But since then, the presence and distribution of circumbinaries has grown to a dozen and some the planets discovered orbiting the two stars have been smaller.  The most recent discovery was announced this week and was made using the Transiting Exoplanet Survey Satellite (TESS) space telescope

The new planet, called TOI (TESS Object of Interest)-1338 b, is about 6.9 times larger than Earth. It orbits its pair of host stars every 95 days, while the stars themselves orbit each other in 15 days.

As is common with binary stars, one is more massive and much brighter than the other (5976 K and 3657 K, respectively, with our sun at  5780 K),  and as the planet orbits around it blocks some of the light from the brighter star.

This transit allows astronomers to measure the size of the planet.  The transit — as scientific luck, or skill, would have it — was first found in the TESS data by a high school student working at NASA with over the summer,  Wolf Cukier

“I was looking through the data for everything the volunteers had flagged as an eclipsing binary, a system where two stars circle around each other and from our view eclipse each other every orbit,” Cukier said. “About three days into my internship, I saw a signal from a system called TOI 1338.”

“At first I thought it was a stellar eclipse, but the timing was wrong. It turned out to be a planet.”

With all of the data available from observations past and current, planet hunting clearly isn’t the scientific Wild West that it used to be — although the results remain often eye-popping and surprising.… Read more

Know Thy Star, Know Thy Planet: How Gaia is Helping Nail Down Planet Sizes

Gaia’s all-sky view of our Milky Way and neighboring galaxies. (ESA/Gaia/DPAC)

Last month, the European Space Agency’s Gaia mission released the most accurate catalogue to date of positions and motions for a staggering 1.3 billion stars.

Let’s do a few comparisons so we can be suitably amazed. The total number of stars you can see without a telescope is less than 10,000. This includes visible stars in both the northern and southern hemispheres, so looking up on a very dark night will allow you to count only about half this number.

The data just released from Gaia is accurate to 0.04 milli-arcseconds. This is a measurement of the angle on the sky, and corresponds to the width of a human hair at a distance of over 300 miles (500 km.) These results are from 22 months of observations and Gaia will ultimately whittle down the stellar positions to within 0.025 milli-arcseconds, the width of a human hair at nearly 680 miles (1000 km.)

OK, so we are now impressed. But why is knowing the precise location of stars exciting to planet hunters?

The reason is that when we claim to measure the radius or mass of a planet, we are almost always measuring the relative size compared to the star. This is true for all planets discovered via the radial velocity and transit techniques — the most common exoplanet detection methods that account for over 95% of planet discoveries.

It means that if we underestimate the star size, our true planet size may balloon from being a close match to the Earth to a giant the size of Jupiter. If this is true for many observed planets, then all our formation and evolution theories will be a mess.

The size of a star is estimated from its brightness. Brightness depends on distance, as a small, close star can appear as bright as a distant giant. Errors in the precise location of stars therefore make a big mess of exoplanet data.


An artist’s impression of the Gaia spacecraft — which is on a mission to chart a three-dimensional map of our Milky Way. In the process it will expand our understanding of the composition, formation and evolution of the galaxy. (ESA/D. Ducros)

This issue has been playing on the minds of exoplanet hunters.

In 2014, a journal paper authored by Fabienne Bastien from Vanderbilt University suggested that nearly half of the brightest stars observed by the Kepler Space Telescope are not regular stars like our sun, but actually are distant and much larger sub-giant stars.… Read more

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