Exoplanets

The first planet around a Sun-like star was confirmed in 1995. More than 5500 are now known. The field moved from establishing existence to characterising populations in about two decades, and the dominant lesson is that the solar system is not typical.

Detection methods

Each method is sensitive to a different region of parameter space, and every statement about the exoplanet population is a statement about the method that found them.

Transit photometry detects the dip as a planet crosses its star. The depth is the area ratio:

Eighty-four parts per million is the measurement that Kepler was built to make, and it required photometric stability far beyond anything achievable through the atmosphere. Transits also require the orbit to be nearly edge-on, so only a small geometric fraction of systems are detectable, roughly 0.5% for an Earth-like orbit.

Radial velocity detects the star’s reflex motion through Doppler shift:

Nine centimetres per second is the precision needed for an Earth analogue, and it is at or beyond the limit of current spectrographs. The obstacle is no longer the instrument but the star: convection, spots and magnetic activity produce apparent velocity shifts of metres per second that mimic or mask planets. Distinguishing stellar noise from planetary signal is the field’s central technical problem.

The population

Three findings overturned expectations formed from the solar system.

Hot Jupiters exist. 51 Pegasi b, the first detection around a Sun-like star, is roughly Jupiter’s mass at 0.05 AU with a four-day orbit. Nothing in solar system experience suggested a gas giant could be there, and it forced the recognition that planets migrate substantially after formation.

The commonest planets have no solar system analogue. Planets between Earth and Neptune in size, super-Earths and mini-Neptunes, are the most abundant class found, and the solar system contains none.

There is a radius valley. The size distribution has a deficit near 1.8 Earth radii, separating rocky super-Earths from gas-enveloped mini-Neptunes. The favoured explanation is atmospheric loss: planets that could not retain a hydrogen envelope against stellar irradiation lost it and shrank, while those that retained it stayed larger. Bodies in between are unstable and rare.

Occurrence rate estimates suggest small planets are common, with of order one planet per star on average and a substantial fraction of M dwarfs hosting roughly Earth-sized planets in their habitable zones.

Atmospheres

Transmission spectroscopy is the workhorse of characterisation. During transit, a small fraction of starlight passes through the planet’s atmosphere, and molecules absorb at characteristic wavelengths. Comparing in-transit and out-of-transit spectra gives the planet’s transmission spectrum.

The effect is tiny, typically tens to hundreds of parts per million on top of an already small transit depth, which is why JWST was transformative here rather than merely useful. Detections now include water, carbon dioxide, methane, sulfur dioxide and evidence of photochemistry.

Emission spectroscopy measures the planet’s own thermal emission by observing the secondary eclipse as it passes behind its star, which constrains temperature structure rather than composition alone. Phase curves, tracking brightness through a full orbit, map day-night temperature contrast and hence atmospheric circulation.

The persistent obstacle is clouds and hazes. High-altitude aerosols mute spectral features, flattening the spectrum and hiding the composition beneath. A substantial fraction of observed planets show flatter spectra than clear-sky models predict.

Habitability

The habitable zone is conventionally the range of orbital distances where liquid water could exist on a rocky planet’s surface given an Earth-like atmosphere, roughly 0.95 to 1.4 AU for the Sun and scaling with stellar luminosity.

The concept is useful and frequently over-interpreted. It says nothing about whether a planet has water, an atmosphere, a magnetic field or a stable climate. Venus and Mars both lie at or near the Sun’s habitable zone boundaries. See Venus for what proximity to habitability does not guarantee.

M dwarfs are the accessible case and the problematic one. They are the most numerous stars, their habitable zones are close in, and the resulting transits are deep and frequent, which makes small planets detectable and characterisable. TRAPPIST-1 hosts seven roughly Earth-sized planets and is the best-studied example. But close-in orbits mean tidal locking, and M dwarfs produce frequent powerful flares that may strip atmospheres entirely. Whether M dwarf planets retain atmospheres at all is an open observational question that JWST is addressing directly.

Biosignatures

A biosignature is an observable feature better explained by life than by anything else. The difficulty is entirely in that final clause.

Oxygen and ozone

On Earth, produced overwhelmingly by photosynthesis. But abiotic photolysis of water followed by hydrogen escape can build oxygen without life, particularly around M dwarfs.

Chemical disequilibrium

Coexisting gases that should react away, such as methane and oxygen together. Stronger than any single gas, because it implies continuous replenishment.

Seasonal variation

Periodic changes in composition correlated with orbital position. Difficult to measure and not yet achievable.

The methodological problem

Every proposed biosignature has a known abiotic pathway. A claim rests on excluding them for a specific planet, which is a much harder argument than making the detection.

The Venus phosphine episode, discussed in Venus, is a useful cautionary case from within the solar system, where far more data was available than for any exoplanet.

Direct imaging

Seeing a planet directly rather than inferring it is the goal, because it works for non-transiting systems and gives a spectrum without needing the star’s light to pass through the atmosphere.

The difficulty is contrast. An Earth-like planet is roughly 10¹⁰ times fainter than its star, separated by a fraction of an arcsecond. Suppressing starlight by that factor at that separation requires coronagraphs or starshades of a precision not yet demonstrated in flight.

Current direct imaging works only for young, massive, widely separated planets that are still hot from formation and therefore self-luminous in the infrared. The Roman Space Telescope will demonstrate an advanced coronagraph, and the Habitable Worlds Observatory concept, prioritised by the 2020 astrophysics decadal survey, is designed around imaging Earth analogues directly.

References

• Perryman, M. The Exoplanet Handbook, 2nd ed., Cambridge University Press, 2018.
• Mayor, M. and Queloz, D. “A Jupiter-mass companion to a solar-type star”, Nature, 378:355, 1995.
• Fulton, B. J. et al. “The California-Kepler Survey III: A Gap in the Radius Distribution of Small Planets”, The Astronomical Journal, 154:109, 2017.
• Kasting, J. F., Whitmire, D. P. and Reynolds, R. T. “Habitable Zones around Main Sequence Stars”, Icarus, 101:108, 1993.
• Meadows, V. S. et al. “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment”, Astrobiology, 18:630, 2018.
• National Academies, Pathways to Discovery in Astronomy and Astrophysics for the 2020s, 2021.