In 1995, when Dr. Michel Mayor and Dr. Didier Queloz of the Geneva Observatory in Switzerland announced their historic discovery of the very first evidence of a distant planet in orbit around a Sun-like star far beyond our own Solar System, many scientists were confused. These observations indicated that there was an extrasolar planet candidate as massive as our own Solar System’s Jupiter (the largest planet in our Sun’s family), orbiting very close to its host star–51 Pegasi. The new planet, named 51 Pegasi b, dwelled a mere 4,300,000 miles from its parent star, which is only a small fraction of the distance between our own Sun and Mercury–the closest planet to our Sun. New theories were hurriedly devised to explain the existence of this gigantic roaster. One theory suggested that 51 Pegasi b actually formed at a distance from its parent star comparable to Jupiter’s average separation from our Star, and then lost energy–spiraling inward, perhaps to be ultimately consumed, by its ferocious parent star. In May 2013, astronomers released a new study, using data from NASA’s highly successful, though ill-fated, Kepler Space Telescope, suggesting that so-called hot Jupiters, like 51 Pegasi b (despite their star-hugging orbits) are not regularly devoured by their stellar parents. Instead, these migrating planets remain in stable orbits for billions of years–until the inevitable does occur, and they finally plunge into their stars!
51 Pegasi b circles around its parent star every 4.2 days. However, existing theories of planetary system formation indicate that giant Jupiter-like planets can only be born at much greater distances from their parent stars. What is the enormous 51 Peg b doing so close to its fiery host star?
51 Peg is a “neighboring” star, dwelling a comparatively trifling 42 light-years from our Solar System. In October, 1995, Dr. Geoffrey W. Marcy and Dr. R. Paul Butler, then at San Francisco State University and the University of California, Berkeley, confirmed the Swiss team’s historic discovery from the Lick Observatory’s three-meter ‘scope poised atop Mount Hamilton in California.
The news was both good and bad. The good news was that one of the most important questions in astronomy–indeed one of the most important questions of humanity as a whole–had at long last been answered: Yes, there are planets orbiting stars that are like our own Sun, and our own Solar System is not unique in the Cosmic scheme of things.
The bad news was that the good news caused widespread befuddlement in the astronomical community. How could this enormous scorching-hot planet have formed so close to its ferociously flaming parent star? After all, 51 Peg b probably bakes at over 1,800 degrees Fahrenheit. This is hot enough to make it glow red like a toaster coil.
New theories were promptly devised by bewildered scientists to explain the existence of this enormous, red-glowing, roasting world that should not be where it obviously was. No one, however, knew whether the bizarre planet had always been this close to its star, or even what it was composed of. Some theorists postulated that the roaster was essentially one enormous ball of molten rock. Others suggested that the planet–like our own Jupiter–was originally a gas giant born about 100 times farther away from its star, and it was hurled toward 51 Peg through a near brush with an undiscovered second planet or sister star.
An alternative theory stated that the roaster had indeed been born at a comparable distance from its star as Jupiter is from our Sun, but it had slowly lost energy by way of interactions with the disk of gas and dust from which it emerged. The doomed baby planet, therefore, spiraled inward towards its hungry parent star from its distant, safe, and considerably cooler place of birth.
According to this rather hideous scenario, 51 Peg b was only one of several doomed planets born in the outer regions of the disk. Most of its sister planets plummeted to a fiery death inside the stellar furnace of 51 Peg. In the case of 51 Peg b, however, good fortune prevailed, and the roasting planet was spared the horrible, fiery fate of its sister planets. The huge planet failed to crash into its cannibalistic parent star just in the nick of time–and, now, it circles its star, fast and close, roasting miserably in its hellish orbit.
This ghastly “death march of the planets” likely requires a few hundred thousand years. The fact that a planet might survive this catastrophe–at least for a time–likely depends, some astronomers explain, on how late it began its march to doom.
If an entire generation of planets somersaulted into the fiery 51 Peg before 51 Peg b came marching along, another generation of planets may now be in the process of spiraling into the heat from still more remote and frigid orbits.
A Bizarre Variety Of Planets
Our Milky Way Galaxy is literally filled to overflowing with a bizarre variety of planets. In addition to the eight major planets dwelling in our Sun’s family–Mercury, Venus, our Earth, Mars, Jupiter, Saturn, Uranus, and Neptune–there are more than 800 planets confirmed to be in orbit around other stars.
Hot Jupiters, also called roasters (for obvious reasons), are gas giants like Jupiter and Saturn, but they hug their stars very closely, literally roasting from the heat of their merciless parent stars.
“The hot Jupiters are beasts to handle. They are not fitting neatly into our models and are more diverse than we thought. We are just starting to put together the puzzle pieces of what’s happening with these planets, and we still don’t know what the final picture will be,” noted Dr. Nikole Lewis in the May 11, 2013 Astrobiology.net. Dr. Lewis is of the Massachusetts Institute of Technology in Cambridge, Massachusetts, as well as the lead author of a paper published in the Astrophysical Journal discussing one example of a hot Jupiter–HAT-P-2b.
Hot Jupiters have a high probability of transiting their stars, which makes them relatively easy to spot by the transit and radial velocity techniques. Indeed, 51 Peg b was spotted by astronomers using the radial velocity technique, which favors the discovery of large planets in tight orbits around their stars. This technique measures a star’s motion toward and away from Earth as it gravitationally responds to a tugging planet or planets. This was the very first successful method astronomers used to spot extrasolar worlds, and more than 400 of the confirmed extrasolar planets were discovered using this technique. The transit method, used by the ill-fated Kepler mission and other space-and ground-based ‘scopes has detected approximately 270 alien worlds by spotting them as they transit–or cross in front of–the fiery face of their stars. Kepler was launched in 2009, and its primary purpose was to catch small Earth-size worlds passing in front of the face of their stars. Alas, it stopped functioning in May 2013, but still left behind a wealth of data for astronomers to scrutinize for years to come.
A leading extrasolar planet hunter, Dr. Sara Seager, noted in the August 2013 issue of Sky & Telescope that “Despite their favorable characteristics for discovery and follow-up study, hot Jupiters are a fairly rare type of planet. Statistical studies point toward an occurrence of only 0.5% to 1% per Sun-like star. Astronomers don’t think hot Jupiters formed at their current locations because there’s not enough material in planet-forming disks that close to a star. The hot Jupiters most likely formed much farther out and migrated inward to their present locations.” Dr. Seager is a professor of planetary science and professor of physics at the Massachusetts Institute of Technology.
Marching To Their Doom?
A study released in May 2013, using data collected from NASA’s Kepler mission, indicates that hot Jupiters, despite their star-hugging orbits, are not devoured by their host stars on a regular basis. Instead, the roasters stay in place, within fairly stable orbits, for billions of years. Alas, the fateful day may nonetheless come when they are at last consumed by their hideous parent stars.
“Eventually, all hot Jupiters get closer and closer to their stars, but in this study we are showing that this process stops before the stars get too close. The planets mostly stabilize once their orbits become circular, whipping around their stars every few days,” Dr. Peter Plavchan explained in a June 6, 2013 NASA Jet Propulsion Laboratory (JPL) Press Release. Dr. Plavchan is of NASA’s Exoplanet Science Institute at the California Institute of Technology (Caltech) in Pasadena, California.
The study, published in May 2013 in the Astrophysical Journal, is the first to show how the weird hot Jupiters stop dead in their tracks during their relentless march towards the furnace of their parent stars, and halt just short of a terrible doom–at least, for a while. Gravitational, or tidal, forces of a parent star serve to circularize and stabilize an inwardly migrating planet’s orbit–and when its orbit at last becomes circular, it halts in its march.
“When only a few hot Jupiters were known, several models could explain the observations. But finding trends in populations of these planets shows that tides, in combination with gravitational forces by often unseen planetary and stellar companions, can bring these giant planets close to their host stars,” explained Dr. Jack Lissauer in the June 6, 2013 JPL Press Release. Dr. Lissauer is a Kepler scientist at NASA’s Ames Research Center in Moffet Field, California. He is not part of the study.
The study answers some questions about how these enormous, unfortunate planets end their weary, ill-fated march. There were a few other theories explaining this mystery floating around prior to the publication of the May 2013 study. One theory suggested that the parent star’s magnetic field stopped the planets from marching onward to their doom, just in the nick of time. A second theory, alternatively, proposed that the planets stopped in their horrible migration because they reached the end of the dusty road in the planet-birthing disk.
“This theory basically said that the dust road a planet travels on ends before the planet falls all the way into the star. A gap forms between the star and the inner edge of the dusty disk where the planets are thought to stop their migration,” study co-author Dr. Chris Bilinski explained in the June 6, 2013 JPL Press Release. Dr. Bilinski is of the University of Arizona in Tucson.
The theory that this new study finds to be correct indicates that a marching planet halts when the parent star’s tidal forces have finally finished their job of circularizing its orbit.
To test these and other theories, the astronomers studied 126 confirmed extrasolar planets and over 2,300 planet candidates. Most of the candidates and some of the confirmed planets were identified by Kepler. The scientists then looked at how the planets’ distance from their parent stars varied depending on the mass of their stars. They discovered that the various theories explaining what halts migrating planets differ in their predictions of how the mass of a star influences the orbit of the planet. The “tidal forces” scenario predicted that the hot Jupiter children of more massive parent stars would circle farther out, on average. The new survey results matched this “tidal forces” theory and even revealed more of a correlation between massive stars and more distant planetary orbits than predicted.
This study may resolve the mystery of what halts the hideous death march of the planets. However, questions still remain. As gas giant planets march inward to their star, it is believed that they sometimes push around the tinier, rocky planets so that they scoot out of their bullying way. This would end any chance of life evolving on those smaller, rocky, and cruelly dislodged planets. Life on Earth was lucky–Jupiter stayed far away from our Sun, and left our planet in life-loving peace!