  [Methods]. We obtained high-precision radial velocities with HARPS on the ESO
3.6 m telescope and determined precise stellar elemental abundances (~0.01 dex)
using MIKE spectra on the Magellan 6.5m telescope. [Results]. Our data indicate
the presence of a planet with a minimum mass of 26 Earth masses around the
solar twin HIP 68468. The planet is a super-Neptune, but unlike the distant
Neptune in our solar system (30 AU), HIP 68468c is close-in, with a semi-major
axis of 0.66 AU, similar to that of Venus. The data also suggest the presence
of a super-Earth with a minimum mass of 2.9 Earth masses at 0.03 AU; if the
planet is confirmed, it will be the fifth least massive radial velocity planet
discovery to date and the first super-Earth around a solar twin. Both
isochrones (5.9 Gyr) and the abundance ratio [Y/Mg] (6.4 Gyr) indicate an age
of about 6 billion years. The star is enhanced in refractory elements when
compared to the Sun, and the refractory enrichment is even stronger after
corrections for Galactic chemical evolution. We determined a NLTE Li abundance
of 1.52 dex, which is four times higher than what would be expected for the age
of HIP 68468. The older age is also supported by the low log(R'HK) (-5.05) and
low jitter. Engulfment of a rocky planet of 6 Earth masses can explain the
enhancement in both lithium and the refractory elements. [Conclusions]. The
super-Neptune planet candidate is too massive for in situ formation, and
therefore its current location is most likely the result of planet migration
that could also have driven other planets towards its host star, enhancing thus
the abundance of lithium and refractory elements in HIP 68468. The intriguing
evidence of planet accretion warrants further observations to verify the
existence of the planets that are indicated by our data and to better constrain
the nature of the planetary system around this unique star.
