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Astronomers Discover ‘Young Jupiter’ Exoplanet
Stony Brook PhD student’s work with recycled planetary dust brings context to the international scientific team’s finding published in Science

The simulated fly-by of the 51 Eridani star and planet system begins with the view of the sky showing the location of the star near the constellation Orion visible in the northern hemisphere winter. The young star 51 Eridani is 100 light-years from the Sun and a Jupiter-like planet is directly imaged in the infrared in an orbit similar in size to the Sun-Saturn distance. The star also has indirect evidence of belts of rocky debris orbiting closer and farther to the star than the new planet. They fly-by ends with a view back toward the sun from the newly discovered planet. Credit : J. Patience & J. Cornelison (ASU) 

STONY BROOK, N.Y. August 13, 2015 – The first planet detected by the Gemini Planet Imager (GPI) from an international team of astronomers, which includes two scientists from the Department of Physics & Astronomy at Stony Brook University, is one outside earth’s solar system at 100 light years away. The exoplanet is being called a “young Jupiter” by the researchers because it shares many characteristics of Jupiter. A paper outlining the full findings is published in Science.

The finding could serve as a decoder ring for astronomers to understand how planets formed around our sun because one of the best ways to learn how our solar system evolved is to look to younger star systems in the earlier phase of development.

Stanimir Metchev, a Physics & Astronomy Professor at Western University in Canada and at Stony Brook University, is a co-investigator on the scientific study, along with Rahul I. Patel, a PhD student in Stony Brook’s Department of Physics & Astronomy. They are both members of the international Gemini Planet Imager Exoplanet Survey (GPIES) team, which is dedicated to imaging and characterising exoplanets, planets discovered outside of earth’s solar system.

The new planet is called 51 Eridani b. The GPI is a new astronomy instrument operated by an international collaboration headed by Bruce Macintosh, a Professor of Physics in the Kavli Institute at Stanford. The exoplanet is the 'faintest' one on record, and also shows the strongest methane signature ever detected on an alien planet, which should yield additional clues as to how the planet formed.

To see the location of 51 Eridani b, see this video image.  For an explanation of the video, see caption text below.

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Location of 51 Eri in the constellation of Eridamus (The River). With a brightness of 5 magnitude in visible and a declination of a -2 degrees, the star is visible with naked eyes.  Credit: Astrostudio.com & Sarah Blunt, SETI Institute

The key to the solar system?

“What makes 51 Eridani particularly interesting is that it also harbours dust and ice in the planetary system," explains Professor Metchev. “These are much like the dust and the ice grains produced by collisions among asteroids and comets in the Solar System."

Metchev's team conducted a study with data from NASA's Wide-field Infrared Survey Explorer (WISE) to search for any thermal glow that such dust and ice can produce.

"We found that 51 Eridani is surrounded by warm dust that indicates the presence of an asteroid belt," says Patel, who led the WISE study and whose previous work identifying recycled planetary dust, known as “debris disks,” around close to a hundred other star systems, puts the discovery of the exoplanet in context.

“Finding dust around a star is like seeing a large signpost that tells us there might be a planet,” he adds. “This is because the dust is usually created when lots of large asteroids collide and destroy each other, usually pushed around by a large planet – like 51 Eridani b.”

Metchev adds, "And more data from the European Space Agency's Herschel Space Observatory reveal that 51 Eridani is also surrounded by a more distant and colder cometary belt, much like the Kuiper Belt of comets beyond Neptune in the Solar System." 

The two belts – the asteroid and the cometary belt around 51 Eridani – fall on either side of the newly discovered planet 51 Eridani b.

"The overall structure bears striking resemblance to our own Solar System, with Jupiter as the most massive planet orbiting between a belt of asteroids and a belt of comets," explains Metchev. "In 51 Eridani, we are therefore seeing what the Solar System resembled at a very young age, around the time when the Earth was still forming."

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Discovery image of the planet 51 Eridani b with the Gemini Planet Imager taken in the near-infrared light on December 18 2014. The bright central star has been mostly removed to enable the detection of the exoplanet one million times fainter.

Credit: J. Rameau (UdeM) and C. Marois (NRC Herzberg)

A clear line of sight

The GPI  was designed specifically for discovering and analyzing faint, young planets orbiting bright stars. NASA's Kepler mission indirectly discovers planets by the loss of starlight when a planet blocks a star.

"To detect planets, Kepler sees their shadow; GPI sees their glow," says Macintosh. "What GPI does is referred to as direct imaging."

The astronomers use adaptive optics to sharpen the image of a star, and then block out the starlight. Any remaining incoming light is then analyzed, the brightest spots indicating a possible planet.

After GPI was installed on the 8-meter Gemini South Telescope in Chile, the team set out to look for planets orbiting young stars. To date, the astronomers have looked at nearly 100 stars.

"51 Eridani is only 20 million years old, a little more massive than our sun – a perfect target," says James Graham, a professor at UC Berkeley and Project Scientist for GPI.

As far as the cosmic clock is concerned, 20 million years is young for a star, and this is exactly what made the direct detection of the planet possible, explains Macintosh.

"When planets coalesce, material falling into the planet releases energy and heats it up. Over the next hundred millions years they radiate that energy away, mostly as infrared light," says Macintosh.  

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Rahul Patel beside his office blackboard featuring notes and formulas related to research on how to identify and characterize the properties of recycled planetary dust.

Once the astronomers zeroed in on the star, they blocked its light and spotted 51 Eridani b orbiting a little farther away from its parent star than Saturn does from the sun. Even though the light from the planet is very faint – nearly a million times fainter than its star – subsequent observations revealed that it is roughly twice the mass of Jupiter. Other directly-imaged planets are five times the mass of Jupiter or more.   

In addition to being the faintest planet ever imaged, it's also the coldest – 400 Celsius (°C), whereas others are around 700 °C – and features the strongest atmospheric methane signal on record. Previous Jupiter-like exoplanets have shown only faint traces of methane, far different from the heavy methane atmospheres of the gas giants in our solar system.

All of these characteristics, the researchers say, point to a planet that is very much what models suggest Jupiter was like in its infancy.

"All of the exoplanets astronomers have imaged before have atmospheres that look like stars – very cool stars, but still stars," says Macintosh, who led the construction of GPI and now leads the survey. "This is the first one that really looks like a planet."

Of course, it's not exactly like Jupiter. The planet is so young and still has a temperature of 400 °C, which is hot enough to melt lead.

"In the atmospheres of the cold giant planets of our solar system carbon is found as methane, unlike most exoplanets where carbon has mostly been found in the form of carbon monoxide. Since the atmosphere of 51 Eridani is also methane rich, it signifies that this planet is well on its way to becoming a cousin of our own familiar Jupiter," says Mark Marley, an astrophysicist at NASA’s Ames Research Center.

The GPI Exoplanet Survey team, led by Macintosh, is now conducting a three-year program to detect new extrasolar planets and to study their home systems.  Using GPI to find more young solar systems such as 51 Eridani will help astronomers understand the formation of our neighbouring planetary systems, and how similar or different they are from our own.