Should a Planet-Hunting Telescope Stare Longer or Move?
After two years staring at one patch of sky, Europe's PLATO telescope can keep staring or move north. This study simulates both for finding Earth-like planets.
In one paragraph
ESA's PLATO telescope will watch one southern field for at least 2 years, then may keep staring or move to a northern field. This preprint simulates both. Moving north finds the most planets overall, because more stars are watched. Staying south finds far more small, long-period planets: predicted small habitable-zone planets rise from 11 after 2 years to 43 after 8 years. Adding old Kepler data in the north barely helps. All results are simulations from predicted noise.
How did researchers compare PLATO's observing strategies?
PLATO, a European Space Agency mission that has not yet launched, has 26 cameras and will observe a fixed southern field covering about 5% of the sky for at least 2 years. Its goal is to find small planets, down to Earth size, on long orbits around Sun-like stars.
The authors used a tool called pesto to inject 2000 simulated planets, with periods from 0.5 to 1600 d and radii from 0.3 to 16 Earth radii, into predicted PLATO noise and real Kepler and TESS observations, and measured how detectable each would be. They then estimated planet yields from occurrence rates, repeating the simulation 100 times.
They compared staring at the southern field for 2, 4 or 8 years, moving to a northern field that overlaps the Kepler field, and combining 2 years of PLATO with existing TESS data.
Which PLATO strategy finds the most Earth-like planets?
Staring longer at the southern field sharply increases the number of small and long-period planets. Predicted small planets in the habitable zone rise from 11±3 after 2 years to 25±5 after 4 years and 43±6 after 8 years. Planets with periods beyond 100 d rise from 421±19 to 1556±39, and the total from 7766±87 to 12873±112.
With only 2 years and at least three transits required, planets beyond about 350 d cannot be detected; 8 years opens periods beyond 1000 d.
Moving north gives the highest total, because more stars are watched, but adding Kepler data barely changes yields of small, long-period planets (9±3 small habitable-zone planets), since Kepler covers only a fraction of the field. TESS data help mainly stars in its continuous viewing zone, extending the baseline for long periods.
Why does PLATO's observing strategy matter?
Small planets on long orbits around Sun-like stars are the least explored part of known planet populations, because they produce small, rare transits. Finding them needs long, precise observations.
The study frames a trade-off: moving to a second field maximises the total number of planets, while a longer stare favours rare small, long-period planets and would let PLATO measure how common such planets are beyond the periods where Kepler's constraints are strongest. A longer stare also helps follow-up measurements from ground telescopes.
A fair reading is that the choice depends on which science goal is given priority.
What are the limits of these PLATO simulations?
Read before you quote it
This is a preprint that has not been peer reviewed, and its limits are clear:
- Simulated only. PLATO has not launched, so all PLATO noise is predicted; stellar activity could make some stars noisier.
- Optimistic geometry. Sensitivity maps assume planets cross the centre of their stars, which the authors call optimistic.
- Extrapolated rates. Occurrence rates for small, long-period planets were extrapolated beyond measured values.
- Self-defined northern sample. The northern priority sample was built by the authors with cuts optimised for the southern field.
The study does not show that PLATO will find an Earth twin, and it does not decide the mission's strategy, which has not been confirmed.
- It does not show that PLATO will find an Earth twin; it estimates sensitivity and expected numbers.
- It does not decide the mission's strategy, which ESA has not yet confirmed.
- Everything is simulated from predicted noise; PLATO has not launched, and real stellar activity may make noise worse for some stars.
- Sensitivities assume planets crossing the star's centre (impact parameter 0), which the authors call optimistic.
- Long-period and small-planet occurrence rates had to be extrapolated beyond what has been measured.
- The northern Prime-like sample was defined by the authors with cuts optimised for the southern field.
Takeaways
- saysStaring at the southern field for eight years instead of two raises the predicted count of small habitable-zone planets from eleven to forty-three.
- saysMoving north finds the most planets overall, but adding old Kepler data barely helps with small, long-period ones.
- saysIt's all simulated from predicted noise, with some optimistic assumptions, so treat the numbers as a forecast.
- explainsA transit is the small dip in a star's light when a planet passes in front of it; small planets need many transits to stand out from the noise.
- interpretsA fair reading is that staying longer in one field trades a smaller total haul for the rare small planets on Earth-like orbits.
Ask the paper
In PaperSays you can ask this paper anything, by text or by saying “Hey Aiden”. Three questions listeners asked:
How many Earth-like planets could PLATO find?
In these simulations, 11 small habitable-zone planets after 2 years of watching one field, rising to 43 after 8 years. Hear the full answer in the app →
Should PLATO move to the northern sky after two years?
Moving finds the most planets overall, but staying south finds more small, long-period planets. Ask in the app →
Does old Kepler data help PLATO?
Only slightly: Kepler covers a small fraction of the northern field and already found most planets there. Ask in the app →
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