Huge Alien Planet Forcing Host Star Age Prematurely

A nearby star is not acting its age, thanks to the influence of a massive exoplanet.

The close-orbiting alien planet, known as WASP-18b, is apparently disrupting the magnetic field of its host star so much that the object is behaving like a much older star, researchers said.

"WASP-18b is an extreme exoplanet," study lead author Ignazio Pillitteri, of the Instituto Nazionale di Astrofisica (INAF)-Osservatorio Astronomico di Palermo in Italy, said in a statement.

"It is one of the most massive hot Jupiters known and one of the closest to its host star, and these characteristics lead to unexpected behaviour. The planet is causing its host star to act old before its time."

Artist's concept depicting the giant alien planet WASP-18b and its star, which are about 330 light-years away. 

Credit: NASA/CXC/M. Weiss

The star WASP-18, which lies about 330 light-years away, is about as massive as our own sun.

The gas giant WASP-18b weighs in at more than 10 times the mass of Jupiter and completes one orbit around the star in less than 23 hours, leading scientists to classify it as a "hot Jupiter."

WASP-18b's tight orbit has led scientists to estimate that it may have only one million years of life or so remaining before it's destroyed by the parent star.

Pillitteri's team targeted WASP-18 with NASA's Chandra X-ray Observatory and found it to be relatively quiet, a characteristic of older stars.

Young stars tend to be more active, with stronger magnetic fields, larger flares and more intense X-ray emission.

Stellar activity is connected to rotation, a process that slows with age.

Observations of WASP-18 using Chandra revealed no X-ray emission.

This by itself would suggest that the star has an age similar to the sun's 5 billion years, researchers said.

However, Pillitteri and his team used other data as well as theoretical models to calculate that WASP-18 is actually just 500 million to 2 billion years old, and thus approximately 100 times less active than a star its age should be.

"We think the planet is aging the star by wreaking havoc on its innards," said co-author Scott Wolk, of the Harvard-Smithsonian Center for Astrophysics (CfA) in Massachusetts.

WASP-18b's strong gravitational pull may be disrupting the star's magnetic field, researchers said.

The planet's tug exerts forces similar to those imposed on Earth's tides by the moon, but on a much larger scale.

Astronomers discover planet that shouldn't be there

This is an artist's conception of a young planet in a distant orbit around its host star. 

The star still harbours a debris disk, remnant material from star and planet formation, interior to the planet's orbit (similar to the HD106906 system). 

Credit: NASA/JPL-Caltech

An international team of astronomers, led by a University of Arizona graduate student, has discovered the most distantly orbiting planet found to date around a single, sun-like star.

It is the first exoplanet – a planet outside of our solar system – discovered at the UA.

Weighing in at 11 times Jupiter's mass and orbiting its star at 650 times the average Earth-Sun distance, planet HD 106906b is unlike anything in our own Solar System and throws a wrench in planet formation theories.

"This system is especially fascinating because no model of either planet or star formation fully explains what we see," said Vanessa Bailey, who led the research.

Bailey is a fifth-year graduate student in the UA's Department of Astronomy.

It is thought that planets close to their stars, like Earth, coalesce from small asteroid-like bodies born in the primordial disk of dust and gas that surrounds a forming star.

However, this process acts too slowly to grow giant planets far from their star. Another proposed mechanism is that giant planets can form from a fast, direct collapse of disk material.

However, primordial disks rarely contain enough mass in their outer reaches to allow a planet like HD 106906b to form.

Several alternative hypotheses have been put forward, including formation like a mini binary star system.

"A binary star system can be formed when two adjacent clumps of gas collapse more or less independently to form stars, and these stars are close enough to each other to exert a mutual gravitation attraction and bind them together in an orbit," Bailey explained.

"It is possible that in the case of the HD 106906 system the star and planet collapsed independently from clumps of gas, but for some reason the planet's progenitor clump was starved for material and never grew large enough to ignite and become a star."

According to Bailey, one problem with this scenario is that the mass ratio of the two stars in a binary system is typically no more than 10-to-1.

"Every new directly detected planet pushes our understanding of how and where planets can form," said co-investigator Tiffany Meshkat, a graduate student at Leiden Observatory in the Netherlands.

"This planet discovery is particularly exciting because it is in orbit so far from its parent star. This leads to many intriguing questions about its formation history and composition."

"Discoveries like HD 106906 b provide us with a deeper understanding of the diversity of other planetary systems."

More information: The research paper, "HD 106906 b: A Planetary-mass Companion Outside a Massive Debris Disk," has been accepted for publication in The Astrophysical Journal Letters and will appear in a future issue. An online version is available for download at arxiv.org/abs/1312.1265