It’s Young, it’s Hot, and it Hangs out with a Star — Meet CI Tau b!
The massive exoplanet CI Tau b is a world brimming over with contradictions. This planet is massive — 11.6 times greater than Jupiter — and it whizzes around its stellar companion once every nine days. But, this world has grown fast for its age — by a long shot.
Parent star to this alien world is CI Tau, just two million years old — making this star a mere toddler in stellar timelines. Any planet (or planets) orbiting this star are unlikely to be older than their parent star, meaning CI Tau b is also likely to be around two million years old. Most theories of planetary formation, however, suggest worlds this size take up to 10 million years to develop.
“The exciting thing is that we are able to detect light directly from the planet, and it’s the first time that’s been done for a close-in planet around a star this young. The most valuable way to learn how planets form is to study planets, like CI Tau b, that are either still forming or have just formed,” stated Christopher Johns-Krull, professor of astronomy and physics at Rice University.
You Think Legos Hurt? Try Stepping on a Prism
Observations of CI Tau b were carried out at the 2.7 meter Harlan J. Smith Telescope and the 4.3 meter Discovery Channel Telescope at the Lowell Observatory. Astronomers utilized the Immersion Grating Infrared Spectrograph (IGRINS) on these instruments to make detailed studies of this fledgling star system.
Breaking light from a distant light source into its component colors, like sunshine passing through a prism, creates spectral lines. Astronomers use this information to determine the temperature, density, and velocity of a star, as well as chemical makeup of planetary atmospheres. Analysis of the mass and brightness of CI Tau b, together with study of carbon monoxide within its atmosphere, told the story of this alien world.
Spectral lines associated with carbon monoxide were studied, attempting to differentiate between signals coming from CI Tau b and those emanating from its parent star. The star in this system is 134 times brighter than the massive planet orbiting it, providing significant challenges to researchers.
“Many of the spectral lines that are in the planet are also in the star. If both the planet and star were stationary, their spectral lines would all blend together, and we wouldn’t be able to tell what was from the star and what was from the planet. But because the planet rapidly orbits the star, its lines shift back and forth dramatically. We can subtract out the star’s lines and see only the lines from the planet. And from those, we can determine how bright the planet is, relative to the star, which tells us something about how it formed,” explained Lisa Prato of Lowell Observatory.
Dilbert: And we know mass creates gravity because more dense planets have more gravity.
Dogbert: How do we know which planets are more dense?
Dilbert: They have more gravity.
Dogbert: That’s circular reasoning.
Dilbert: I prefer to think of it as having no loose ends.
- Scott Adams, Dilbert
The Best-Laid Planets…
Just a few short decades ago, most astronomers believed solar systems centered on alien stars would be arranged much like our own — small, rocky worlds near the star, with large gas giants placed further out. What they found, however, is that exoplanets are found in a wide variety of arrangements, including a few systems featuring “hot Jupiters,” massive worlds orbiting extremely close to their sun.
Massive planets like Jupiter and Saturn were believed to be formed far from stars, over the course of 10 million years or more. The discovery of hot Jupiters led to new theories of planetary formation and development. New research suggests that massive stars are more likely than smaller stars to be accompanied by such gargantuan planetary companions.
There may also be a “hot-spot,” located between three and 10 times the distance Earth orbits from the Sun, where the largest planets are most likely to be found. In our own solar system, Jupiter currently orbits in that range — 5.2 times more distant from the Sun than our own world. Compelling evidence suggests, however, that Jupiter may have, long ago, migrated around the Solar System.
“Direct observational evidence of the mass and brightness of CI Tau b is particularly useful because we also know it orbits a very young star. Most of the hot Jupiters we’ve found are orbiting middle-aged stars. CI Tau’s age gives a tight constraint for putting models to the test: Can they produce a planet this bright and this massive in so little time?” speculated Laura Flagg, doctoral student at Rice University.
Our solar system is over four billion years old, 2,000 times the age of the CI Tau system. If our sun and family of planets were 50 years old, this infant star system would be less than 10 days of age. Gas and dust left over from the formation of the star is still spread through the inner regions of the CI Tau system.
“This result is unique because it demonstrates that a giant planet can form so rapidly that the remnant gas and dust from which the young star formed, surrounding the system in a Frisbee-like disc, is still present,” said explained Prato, following the 2016 discovery of the exoplanet.
Located 450 light years from Earth, CI Tau is seen in the constellation of Taurus the Bull. This exoplanet was discovered in 2016, by astronomers using the Harlan J. Smith Telescope (part of the McDonald Observatory) managed by the University of Texas at Austin. Starting in 2004, astronomers studying CI Tau uncovered signs of the gravitational pull of an unseen object on the star — the first evidence of the exoplanet CI Tau b.
Currently, around 4,000 planets are known around other stars, and data is being examined for an additional 3,000 exoplanets. Since the discovery of the first exoplanet in 1992, the number of known worlds beyond our Solar System has doubled every 27 months.