Exoplanet ‘Young Jupiter’ 51 Eridani b

An illustration showing the distant exoplanet 51 Eridani b, nicknamed "Young Jupiter". Source: SETIKing of Planets

Astronomers have found the smallest exoplanet yet to be directly photographed by a telescope on Earth.  A methane-shrouded gas giant.  A young Jupiter…

Whereas a star is a celestial body sustained by nuclear fusion at its core, a planet’s interior is always too cool for nuclear fusion.  But sometimes nuclear fusion fails to start and there remains a giant mass of cool gas held together by gravity.  Not quite a planet.  Not yet a star.  Jupiter is one of those half-way stellar oddities.

 

A photographic animation of Jupiter, observed in its infrared spectrum.
Jupiter’s infra-red spectrum Source: NASA images

Jupiter

With dozens of moons and an enormous magnetic field, Jupiter – the most massive planet in our solar system – forms a kind of miniature solar system.  Jupiter resembles a star in its composition, but it did not grow quite big enough and failed to ignite nuclear fusion.  The planet’s swirling cloud stripes are punctuated by massive storms such as the Great Red Spot, which has raged for hundreds of years.

Jupiter is a gas-giant planet and it does not have a solid surface.  However, it is predicted that Jupiter has an inner, solid core about the size of the Earth.  Its atmosphere is mostly made up of molecular hydrogen (H2) and helium (He).

Jupiter is the fifth planet from the Sun at a distance of about 778 million km (484 million miles) or 5.2 AU.  It completes its orbit around the Sun in about 12 Earth years (4,333 Earth days) – a year in Jovian time.

 

A Host of Exoplanets

Once upon a time, extrasolar planets were sheer speculation.  The very term “exoplanet” was a scientific novelty.  An exoplanet or extrasolar planet, is a planet located outside the Solar System

Then, one was found.

HD 114762 b is certainly the first extrasolar planet ever detected in 1989, later confirmed in 1991, but it was not going to be the last one.  In 1992, two exoplanets were found orbiting the pulsar PSR B1257+12.  The ‘companion’ planet orbits its star every 83.9 days – a planetary orbit similar to that of Mercury, but with twice its eccentricity.

The exoplanet had become a reality.  Still, it was a rarity.  A mere space oddity…

In 1995, the first confirmed exoplanet orbiting a main-sequence star was discovered, when a giant planet was found in a four-day orbit around nearby main-sequence yellow dwarf star 51 Pegasi.

Over the last 20 years, astronomers have confirmed the existence of over 1,800 exoplanets – alien worlds outside our own Solar system, gravitating around faraway stars.  More than 1,000 confirmed to-date were discovered by NASA’s Kepler Space Telescope.  As of August 14, 2015, astronomers have identified 1,946 such planets (in 1,230 different planetary systems and 487 multiple planetary systems).

Exoplanets are very dim compared to their host starsAlthough some exoplanets have been imaged directly by telescopes, the vast majority of them was detected through indirect methods such as the transit method and the radial-velocity methodAs such, the exoplanets directly imaged until now were large, weighing at least five times the mass of our Jupiter.

A photograph of the Gemini South observatory dome, pictured against the background of the Chilian Andes, in South America.
Gemini South and the Chilian Andes.  With an 8-metre diameter of the primary mirror, the Gemini South Telescope in Chile is one of the largest telescopes in the World. Source: Centre for Planetary Science and Exploration

The Kepler Space Telescope indirectly finds the planets by detecting a loss of starlight as a planet passes in front of its star

Many of these worlds are quite unlike any planets in Earth’s solar system.  These so-called “hot Jupiters” are gas giants in orbit around their host stars, more closely than Mercury does the Sun.

But this new world, 51 Eridani b, was discovered using the Gemini Planet Imager at the Gemini South telescope in Chile, which directly detects exoplanets by looking for light emitted by the planets themselves.

 

Discovered by Gemini

The Gemini Planet Imager (GPI) is one of a new generation of astrophysical instruments designed specifically for discovering and analysing small, faint, young planets orbiting relatively close to their bright host stars.  The GPI’s technology rely on deformable mirrors.

The astronomers used adaptive optics to sharpen the image of a star, and then block out the starlight.  Any remaining incoming light was then analysed, with the brightest spots indicating a possible planet.

Finding out about this alien world could provide us with clues about the formation of our Solar System, which has an unusual lay-out.  Previous Jupiter-like exoplanets have shown only faint traces of methane, making them very different from the heavy methane atmospheres of gas giants in our Solar System.

The new exoplanet shows the strongest methane signature ever detected on an alien world.  The astronomers also detected water, using spectrometry.

The discovery image of exoplanet 51 Eridani b.
51 Eridan b discovery image.  Source: space.com

Planet 51 Eridani b, or 51 Eri b for short, is only 20 million years old – a toddler by astronomical standards.  By comparison, the planets in our Solar System are 4.5 billion years old.

The gas giant is roughly twice the mass of Jupiter.

51 Eridani b is more than a million times fainter than its star, and it still glows from the heat of its creation.  The planet orbits its star at a distance of about 13 astronomical units, or about 13 times the span between Earth and the Sun.  This is between the distances of Saturn and Uranus from the sun.

 

1 AU = one astronomical unit – about 93 million miles (150 million kilometres) = Earth-Sun distance.

 

Direct imaging of exoplanets provides more than just photographs.  The wavelengths of light a planet gives off can reveal a great deal of information, such as its chemical composition.  The researchers found that 51 Eri b’s atmosphere is dominated by methane, much like Jupiter’s.

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 Eri b is also methane rich, it signifies that this planet is well on its way to becoming a cousin of our own familiar Jupiter.

Astronomers think that the gas giants in the solar system arose by growing large rocky cores over a few million years, then gravitationally pulling in huge amounts of hydrogen and other gases around themselves to form atmospheres.  However, until now, directly-imaged gas giants around other stars have been relatively hot at about 650 degrees Celsius (1,200 degrees Fahrenheit) – compared to the temperature of Jupiter, which is about minus 145°C (-234°F).

This heat suggests these far-off gas giants might have formed much faster than the solar system’s gas giants, with their ingredients gravitationally attracting together and collapsing rapidly to make searing-hot planets.

At about 430°C (800°F), the scientists found that 51 Eri b is relatively cool.  While that temperature is hot enough to melt lead, it is cool enough to be consistent with the cold-start-formation mechanism, according to the researchers.

A representation of our Solar system, featuring the Sun at its centre, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.Previous search methods could not find systems like our own, with small, rocky worlds close to their star and large, gas giants at large distances like Jupiter and Saturn.  The search for large planets at large separations from their star is exactly the goal of GPI.

These solar systems are likely much more similar to our own…