The Dilemma of Magnetospheres
Astronomers have been trying to understand planetary magnetic fields, called magnetospheres. In our solar system, there is no definitive explanation which will determine which planets will generate magnetospheres. For instance, Venus, approximately the same size and composition as Earth, does not have one.
Astronomers have been collecting data on exoplanet magnetic fields to see if researchers could find some common features of other planets in other star systems. This may give us a wider view of our solar system and answer some nagging questions about why does one world have a magnetic field while the other does not? Exoplanet magnetospheres are very faint and difficult to detect, so collecting this data is quite a challenge. But last year two separate teams have discovered an apparent magnetic field signature produced by a planet in a red dwarf system 12 light-years away called YZ Ceti b. This planet is slightly smaller than earth and probably too hot for life. However, studying planets like this might tell us more about how magnetospheres are created and how they impact a planet’s evolution, including its suitability for life. Magnetic fields play a crucial role in atmospheric retention. The question is: how common are planetary magnetic fields similar to ours?
Magnetic Signals
In our solar system, Earth and the four giant planets — Jupiter, Saturn, Uranus and Neptune — have significant magnetospheres. Mercury has only a faint field, and Mars very likely had a more robust field in the past, which it lost for reasons that aren’t completely understood.
The planet's dynamo is the process creating the magnetic field is composed of molten metal spinning in the core, producing electrical currents. Earth and the four gas giants have a strong enough force to deflect solar winds and cosmic rays that could dissipate the planets’ atmospheres into space. It is a powerful radiation shield, important to protect life.
Perhaps 5,000 of the known exoplanets have magnetic fields, but confirming this is difficult. As early as the 1970s, astronomers hypothesized that when a planetary magnetosphere of a planet interacts with its star, it might produce an detectable spike in low-frequency radio waves coming from the star. The pattern of the planet's presence could be discovered from the timing of the spikes which would indirectly determine a planet’s location in its orbit. The technique was challenging; no one has made a detection of an exoplanet's magnetic field so far. There have been possible candidates, though, such as one in the in the Tau Boötes system. This is a planet similar to Neptune whose emissions indicated a possible magnetosphere. But no findings were conclusive, and none were of rocky planets similar to Earth.
More Recent Discovery
In 2017, astronomers found the ideal start system they needed for the kind of indirect observation they had been hypothesizing about for almost five decades. This was a system with three rocky planets orbiting around the red dwarf YZ Ceti, very close in cosmic terms. Being so close to our own makes its planets easier to detect — especially YZ Ceti b, the innermost planet. Also, red dwarfs typically have stronger magnetospheres than our own sun, which makes it easier to identify the unique pattern of an orbiting planet’s magnetic field.
Two teams have now turned up evidence of a magnetic field made by YZ Ceti b, spotting periodic bursts of radio waves that apparently occurred when YZ Ceti b reached the same location in its two-day orbit around the star. Using the Very Large Array in New Mexico, one team worked out that the planet would need a magnetic field strength similar to Earth’s to cause the strength of the signal they detected. The other team posted their findings shortly after. They recorded similar observations of periodic radio spike patterns using the Giant Metrewave Radio Telescope in India.
The results are promising, but the scientists do not consider it an iron-clad confirmation yet. A more definitive detection will need more observations of the star and the periodic radio spikes. They are hoping that similar observations can be attempted on the TRAPPIST-1 system of seven worlds orbiting a red dwarf 40 light-years from Earth, or the red dwarf Proxima Centauri. Proxima Centauri is the closest star to Earth at only 4.25 light-years away, and likely hosts a rocky planet.
Many Unanswered Questions
Finding magnetospheres in other star systems is crucial for understanding how common they are and how planets make magnetism. “We don’t really have an amazing understanding of how these things are generated on planets,” said Robert Kavanagh, an astronomer at the Netherlands Institute for Radio Astronomy.
In our solar system, a dynamo seems to be key. But there may be other ways to generate a planetary magnetic field, especially in “super-Earths” — worlds significantly larger than Earth, one of the most common type of exoplanet spotted so far. Miki Nakajima, a planetary scientist at the University of Rochester, is investigating whether heat fluctuations inside a planet could do the job for worlds that have molten interiors and no solid core. “I’m interested in whether a magma ocean can produce a magnetic field,” she said, noting that “magma oceans should be pretty common in super-Earths.”
Source: Quanta Magazine
