MAGNETIC FIELDS OF EXOPLANETS. FEATURES AND

DETECTION

UCM, 27th May 2014Enrique Blanco Henríquez

OUTLINE

• Magnetospheres of Earth-like exoplanets• Dynamo mechanism

• Hot Jupiters magnetospheres• Atmospheric escape from Hot Jupiters• Magnetodisks

• Radio emission related to magnetic fields• far-UV transits

• Bow-shocks

Magnetospheres in Earth-like exoplanets• Magnetic field sustained by a dynamo mechanism• In spite of major differences in structure, composition, and history, most of these dynamos

are thought to be maintained by similar mechanisms: thermal and compositional convection in electrically conducting fluids in the planet interiors

• Tarter et al. 2007 and Scalo et al. 2007 recommended M-dwarfs as best targets to search for exo-Earths.

• M-dwarfs more active than Sun-like stars planets will be exposed to denser winds. However, Planets are tidally locked, are in synchronous rotation and have weak magnetic moments (maybe not as weak as we thought)

• Early model attempts

• Olson & Christensen (2006), independent of rotation rate

Magnetospheres in Earth-like exoplanets

• Nowadays, it is not know if F and D change with time• However, rotation rate can play an important role in the nature of the magnetic field

- Fast rotators dipole - Slow rotators multipole

Magnetospheres in Earth-like exoplanets

• Magnetic moment depends on its rotation rate, but also on it’s chemical composition and the efficiency of convection in its interior (F)

• Ω only marks if the dynamo is dipolar or multipolar, but magnetic moment strength will not explicitly depend on rotation.

• Planets under extreme conditions, i.e. highly inhomogeneous heating or under very strong stellar winds, may have their magnetic field affected.

• This is still work in progress and a better understanding of the interior structure and energy transportation mechanisms in rocky planets is still necessary.

Hot Jupiters Magnetospheres

usual Giants

Super-Earths

Hot Jupiters

Hot Jupiters Magnetospheres

• Upper atmospheres subjected to intense heating and tidal forces• Magnetic pressure dominates gas pressure (gas rarified)• High temperatures generated by EUV heating

• Soft X-ray and EUV induced expansion of the upper atmosphere

• Thermal escape: • Jeans escape – particles from tails• Hydrodynamic escape – all particles

• Non-thermal escape:• Ion pick-up• Sputtering• Photo-chemical energizing & escape• Electromagnetic ion acceleration

Hot Jupiters Magnetospheres- importance of magnetodisk

• Huge amount of Hot Jupiters are efficiently protected against extreme plasma and radiation conditions.

• All estimations were based on too simplified model.• It was considered a planetary dipole dominated magnetosphere only• Dipole magnetic field balances stellar wind ram pressure

• However, big M is needed for efficient protection:

big tidal locking small M

• Specifically for close-in exoplanets, new model is required• Strong mass loss of a planet should lead to formation of a plasma disk • A magnetodisk domaining magnetosphere• More complete planetary magnetosphere model, including the whole complex of the magnetospheric electric current systems

Hot Jupiters Magnetospheres- importance of magnetodisk

• Formation of magnetodisk for Hot Jupiters

• “Sling” model:

Dipole magnetic field drives plasma in

co-rotation regimen inside the

Alfvenic surface.

• “material-escape driven” modelsHydrodynamic escape of plasma.Dipolar magnetic field provoques a charge separation which causes an electric field Hall current inequator plane.

Hot Jupiters Magnetospheres- importance of magnetodisk

• Paraboloid Magnetospheirc Model (PMM) for Hot Jupiters • Key assumption: magnetopause is approximated by paraboloid of revolution along

planet-star line

- Planetary magnetic dipole

- Magnetotail

- Magnetodisk

- Magnetopause currents

- Magnetic field of stellar wind

Radio emission from exoplanets

• Interaction between the stellar wind and the magnetised planet provoques a reconnection that releases energetic electrons: radio emission

Detection of cyclotron radio emission (CRE) would prove thatthe exoplanet is magnetised

Electron cyclotron emission frequency:

Radio Bode’s Law

The radio flux observed at the Earth

Radio emission from exoplanetsOptimal dynamos in the cores of terrestrial exoplanets:Magnetic field generation and detectability. Driscoll and Olson 2011

- CRE for 32% and 65% CMF exoplanets

- The ionospheric cutoff at 10 MHz sets the lower frequency limit for ground-based radio telescopes

such as LOFAR. - LOFAR (LOw Frequency ARray)

- It’s is possible to detect CRE?- Small fluxes - To be detectable with LOFAR, emission power must increase by a 1e3 factor

Measuring planetary magnetic field with transition observations

• Asymmetry between the ingress and egress times can be observed in the near-UV light curve compared to the optical observations (eg. WASP-12b)

• Led to suggest the existance of a bow-shock surrounding the planet’s atmosphere.

• For a shock to develop, the relative velocity between the planet and the stellar corona must be greater than local sound speed

• For a shock to be detected, it must compress the local plasma to a density high enough. For a hydrostatic, isothermal corona, the local density is

• Suppose that coronal material from the star is not magnetically confined, so it can escape in the form of a wind

• Monte Carlo simulations for WASP-12b (early ingress)

Measuring planetary magnetic field with transition observations

• Measuring the planetary magnetic field (Vidotto et al. 2010)

Measuring planetary magnetic field with transition observations